c7n-mailer: Custodian Mailer

A mailer implementation for Custodian. Outbound mail delivery is still somewhat organization-specific, so this at the moment serves primarily as an example implementation.

The Cloud Custodian Mailer can now be easily run in a Docker container. Click here for details.

Message Relay

Custodian Mailer subscribes to an SQS queue, looks up users, and sends emails via SES, Slack messages, and/or notifications to DataDog or Splunk. Custodian Lambda and instance policies can send messages to the SQS queue. SQS queues should be cross-account enabled for sending between accounts.


Our goal in starting out with the Custodian mailer is to install the mailer, and run a policy that triggers an email to your inbox.

  1. Install the mailer on your laptop (if you are not running as a Docker container

    • or use pip install c7n-mailer

  2. In your text editor, create a mailer.yml file to hold your mailer config.

  3. In the AWS console, create a new standard SQS queue (quick create is fine). Copy the queue URL to queue_url in mailer.yml.

  4. In AWS, locate or create a role that has read access to the queue. Grab the role ARN and set it as role in mailer.yml.

There are different notification endpoints options, you can combine both.


Make sure your email address is verified in SES, and set it as from_address in mailer.yml. By default SES is in sandbox mode where you must verify every individual recipient of emails. If need be, make an AWS support ticket to be taken out of SES sandbox mode.

Your mailer.yml should now look something like this:

queue_url: https://sqs.us-east-1.amazonaws.com/1234567890/c7n-mailer-test
role: arn:aws:iam::123456790:role/c7n-mailer-test
from_address: you@example.com

You can also set region if you are in a region other than us-east-1 as well as lambda_tags to give the mailer tags.

region: us-east-2
  owner: ops

Now let’s make a Custodian policy to populate your mailer queue. Create a test-policy.yml file with this content (update to and queue to match your environment)

  - name: c7n-mailer-test
    resource: sqs
      - "tag:MailerTest": absent
      - type: notify
        template: default
        priority_header: '2'
        subject: testing the c7n mailer
          - you@example.com
          type: sqs
          queue: https://sqs.us-east-1.amazonaws.com/1234567890/c7n-mailer-test


The standard way to do a DataDog integration is use the c7n integration with AWS CloudWatch and use the DataDog integration with AWS to collect CloudWatch metrics. The mailer/messenger integration is only for the case you don’t want or you can’t use AWS CloudWatch, e.g. in Azure or GCP.

Note this integration requires the additional dependency of Datadog Python bindings:

pip install datadog

Your mailer.yml should now look something like this:

queue_url: https://sqs.us-east-1.amazonaws.com/1234567890/c7n-mailer-test
role: arn:aws:iam::123456790:role/c7n-mailer-test

(Also set region if you are in a region other than us-east-1.)

Now, let’s make a Custodian policy to populate your mailer queue. Create a test-policy.yml:

  - name: c7n-mailer-test
    resource: ebs
     - Attachments: []
      - type: notify
          - datadog://?metric_name=datadog.metric.name&metric_value_tag=Size
          type: sqs
          queue: https://sqs.us-east-1.amazonaws.com/1234567890/c7n-mailer-test

There is a special to format that specifies Datadog delivery and it includes the Datadog configuration via URL parameters:

  • metric_name: is the name of the metrics send to DataDog.

  • metric_value_tag: by default the metric value to send to DataDog is 1, but if you want to use one of the tags returned in the policy, you can set it with the attribute metric_value_tag. For example, in the test-policy.yml policy, the value used is the size of the EBS volume. The value must be a number and it will be transformed to a float value.


The Custodian mailer supports Slack messaging as a separate notification mechanism for the SQS transport method. To enable Slack integration, you must specify a Slack token in the slack_token field in your mailer.yml file. For example:

queue_url: https://sqs.us-east-1.amazonaws.com/1234567890/c7n-mailer-test
role: arn:aws:iam::123456790:role/c7n-mailer-test
slack_token: xoxo-token123

To enable Slack messaging, several unique fields are evaluated in the policy, as shown in the below example:

  - name: c7n-mailer-test
    resource: ebs
     - Attachments: []
      - type: notify
        slack_template: slack
        slack_msg_color: danger
          - slack://owners
          - slack://foo@bar.com
          - slack://#custodian-test
          - slack://webhook/#c7n-webhook-test
          - slack://tag/resource_tag
          - https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX
          type: sqs
          queue: https://sqs.us-east-1.amazonaws.com/1234567890/c7n-mailer-test

Slack messages support the use of a unique template field specified in slack_template. This field is unique and usage will not break existing functionality for messages also specifying an email template in the template field. This field is optional, however, and if not specified, the mailer will use the default value slack_default.

The unique template field slack_msg_color can be used to specify a color border for the Slack message. This accepts the Slack presets of danger (red), warning (yellow) and good (green). It can also accept an HTML hex code. See the Slack documentation for details.

Note: if you are using a hex color code, it will need to be wrapped in quotes like so: slack_msg_color: '#4287f51'. Otherwise, the YAML interpreter will consider it a comment.

The Slack integration for c7n-mailer supports several flavors of messaging, listed below. These are not mutually exclusive and any combination of the types can be used, but the preferred method is incoming webhooks.

Requires slack_token Key Type Notes
No https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX string (PREFERRED) Send to an incoming webhook (the channel is defined in the webhook)
Yes slack://owners string Send to the recipient list generated within email delivery logic
Yes slack://foo@bar.com string Send to the recipient specified by email address foo@bar.com
Yes slack://#custodian-test string Send to the Slack channel indicated in string, i.e. #custodian-test
No slack://webhook/#c7n-webhook-test string (DEPRECATED) Send to a Slack webhook; appended with the target channel. IMPORTANT: This requires a slack_webhook value defined in the mailer.yml.
Yes slack://tag/resource-tag string Send to target found in resource tag. Example of value in tag: https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX

Slack delivery can also be set via a resource’s tag name. For example, using slack://tag/slack_channel will look for a tag name of slack_channel, and if matched on a resource will deliver the message to the value of that resource’s tag:

slack_channel:https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX slack_channel:custodian-test owner:foo@bar

Delivery via tag has been tested with webhooks, but it should support all delivery methods.

Splunk HTTP Event Collector (HEC)

The Custodian mailer supports delivery to the HTTP Event Collector (HEC) endpoint of a Splunk instance as a separate notification mechanism for the SES transport method. To enable Splunk HEC integration, you must specify the URL to the HEC endpoint as well as a valid username and token:

queue_url: https://sqs.us-east-1.amazonaws.com/1234567890/c7n-mailer-test
role: arn:aws:iam::123456790:role/c7n-mailer-test
splunk_hec_url: https://http-inputs-foo.splunkcloud.com/services/collector/event
splunk_hec_token: 268b3cc2-f32e-4a19-a1e8-aee08d86ca7f

To send events for a policy to the Splunk HEC endpoint, add a to address notify action specifying the name of the Splunk index to send events to in the form splunkhec://indexName:

  - name: c7n-mailer-test
    resource: ebs
     - Attachments: []
      - type: notify
          - splunkhec://myIndexName
          type: sqs
          queue: https://sqs.us-east-1.amazonaws.com/1234567890/c7n-mailer-test

The splunkhec://indexName address type can be combined in the same notify action with other destination types (e.g. email, Slack, DataDog, etc).

Now run:

c7n-mailer --config mailer.yml --update-lambda && custodian run -c test-policy.yml -s .

Note: You can set the profile via environment variable e.g. export AWS_DEFAULT_PROFILE=foo

You should see output similar to the following:

(env) $ c7n-mailer --config mailer.yml --update-lambda && custodian run -c test-policy.yml -s .
DEBUG:custodian.lambda:Created custodian lambda archive size: 3.01mb
2017-01-12 07:55:16,227: custodian.policy:INFO Running policy c7n-mailer-test resource: sqs region:default c7n:
2017-01-12 07:55:16,229: custodian.policy:INFO policy: c7n-mailer-test resource:sqs has count:1 time:0.00
2017-01-12 07:55:18,017: custodian.actions:INFO sent message:dead-beef policy:c7n-mailer-test template:default count:1
2017-01-12 07:55:18,017: custodian.policy:INFO policy: c7n-mailer-test action: notify resources: 1 execution_time: 1.79
(env) $

Check the AWS console for a new Lambda named cloud-custodian-mailer. The mailer runs every five minutes, so wait a bit and then look for an email in your inbox. If it doesn’t appear, look in the lambda’s logs for debugging information. If it does, congratulations! You are off and running with the Custodian mailer.

Usage & Configuration

Once installed you should have a c7n-mailer executable on your path:

(env) $ c7n-mailer
usage: c7n-mailer [-h] -c CONFIG
c7n-mailer: error: argument -c/--config is required
(env) $

Fundamentally, what c7n-mailer does is it deploys a Lambda function (using Mu) based on the configuration you specify in a YAML file. Here is the schema to which the file must conform and here is a description of the options:

Required? Key Type Notes
queue_url string the queue to listen to for messages
from_address string default from address
endpoint_url string SQS API URL (for use with VPC Endpoints)
contact_tags array of strings tags that we should look at for address information
email_base_url string Base URL to construct a valid email address from a resource owner
additional_email_headers object Additional email headers.
org_domain string Domain to add to AWS usernames to construct email addresses.

Standard Lambda Function Config

Required? Key Type
role string
dead_letter_config object
memory integer
region string
runtime string
security_groups array of strings
subnets array of strings
timeout integer
lambda_name string
lambda_description string
lambda_tags object
lambda_schedule string

Standard Azure Functions Config

Required? Key Type Notes
function_properties object Contains appInsights, storageAccount and servicePlan objects
appInsights object Contains name, location and resourceGroupName properties
storageAccount object Contains name, location and resourceGroupName properties
servicePlan object Contains name, location, resourceGroupName, skuTier and skuName properties
name string
location string Default: west us 2
resourceGroupName string Default cloud-custodian
skuTier string Default: Basic
skuName string Default: B1

Mailer Infrastructure Config

Required? Key Type Notes
cache_engine string cache engine; either sqlite or redis
cross_accounts object account to assume back into for sending to SNS topics
debug boolean debug on/off
ldap_bind_dn string eg: ou=people,dc=example,dc=com
ldap_bind_user string eg: FOO\BAR
ldap_bind_password secured string ldap bind password
ldap_bind_password_in_kms boolean defaults to true, most people (except capone) want to set this to false. If set to true, make sure ldap_bind_password contains your KMS encrypted ldap bind password as a base64-encoded string.
ldap_email_attribute string
ldap_email_key string eg 'mail'
ldap_manager_attribute string eg 'manager'
ldap_uid_attribute string
ldap_uid_regex string
ldap_uid_tags string
ldap_uri string eg 'ldaps://example.com:636'
redis_host string redis host if cache_engine == redis
redis_port integer redis port, default: 6369
ses_region string AWS region that handles SES API calls
ses_role string ARN of the role to assume to send email with SES

SMTP Config

Required? Key Type Notes
smtp_server string to configure your lambda role to talk to smtpd in your private vpc, see here
smtp_port integer smtp port (default is 25)
smtp_ssl boolean this defaults to True
smtp_username string
smtp_password secured string

If smtp_server is unset, c7n_mailer will use AWS SES or Azure SendGrid.

DataDog Config

Required? Key Type Notes
datadog_api_key string DataDog API key.
datadog_application_key string Datadog application key.

These fields are not necessary if c7n_mailer is run in a instance/lambda/etc with the DataDog agent.

Slack Config

Required? Key Type Notes
slack_token string Slack API token

SendGrid Config

Required? Key Type Notes
sendgrid_api_key secured string SendGrid API token

Microsoft Graph Config

Required? Key Type Notes
graph_token_endpoint string Graph token endpoint
graph_sendmail_endpoint string Graph sendmail endpoint
graph_client_id string Client ID
graph_client_secret secured string Client secret

Splunk HEC Config

The following configuration items are all optional. The ones marked “Required for Splunk” are only required if you’re sending notifications to splunkhec:// destinations.

Required for Splunk? Key Type Notes
splunk_hec_url string URL to your Splunk HTTP Event Collector endpoint
splunk_hec_token string Splunk HEC authentication token for specified username
splunk_remove_paths array of strings List of RFC6901 JSON Pointers to remove from the event, if present, before sending to Splunk
splunk_actions_list boolean If true, add an actions list to the top-level message sent to Splunk, containing the names of all non-notify actions taken
splunk_max_attempts integer Maximum number of times to try POSTing data to Splunk HEC (default 4)
splunk_hec_max_length integer Maximum data length that Splunk HEC accepts; an error will be logged for any message sent over this length
splunk_hec_sourcetype string Configure sourcetype of the payload sent to Splunk HEC. (default is '_json')

SDK Config

Required? Key Type Notes
http_proxy string
https_proxy string
profile string

Secured String

In order to ensure sensitive data is not stored plaintext in a policy, c7n-mailer supports secured strings. You can treat it as a regular string or use secured string features.


You can use KMS to encrypt your secrets and use encrypted secret in mailer policy. Custodian tries to decrypt the string using KMS, if it fails c7n treats it as a plaintext secret.

 plaintext_secret: <raw_secret>
 secured_string: <encrypted_secret>


You can store your secrets in Azure Key Vault secrets and reference them from the policy.

  plaintext_secret: <raw_secret>
    type: azure.keyvault
    secret: https://your-vault.vault.azure.net/secrets/your-secret

Note: secrets.get permission on the KeyVault for the Service Principal is required.


You can store your secrets as GCP Secret Manager secrets and reference them from the policy.

  plaintext_secret: <raw_secret>
    type: gcp.secretmanager
    secret: projects/12345678912/secrets/your-secret

An example of an SMTP password set as a secured string:

    type: gcp.secretmanager
    secret: projects/59808015552/secrets/smtp_pw

Note: If you do not specify a version, /versions/latest will be appended to your secret location.

Configuring a policy to send email

Outbound email can be added to any policy by including the notify action.

  - name: bad-apples
    resource: asg
     - "tag:ASV": absent
      - type: notify
        template: default
        template_format: 'html'
        priority_header: '1'
        subject: fix your tags
          - resource-owner
          - foo@example.com
          type: sqs
          queue: https://sqs.us-east-1.amazonaws.com/80101010101/cloud-custodian-message-relay

So breaking it down, you add an action of type notify. You can specify a template that’s used to format the email; customizing templates is described below.

The to list specifies the intended recipient for the email. You can specify either an email address, an SNS topic, a Datadog Metric, or a special value. The special values are either

  • resource-owner, in which case the email will be sent to the listed OwnerContact tag on the resource that matched the policy, or

  • event-owner for push-based/realtime policies that will send to the user that was responsible for the underlying event.

  • priority_header to indicate the importance of an email with headers. Different emails clients will display stars, exclamation points or flags depending on the value. Should be an string from 1 to 5.

Both of these special values are best effort, i.e., if no OwnerContact tag is specified then resource-owner email will not be delivered, and in the case of event-owner an instance role or system account will not result in an email.

The optional owner_absent_contact list specifies email addresses to notify only if the resource-owner special option was unable to find any matching owner contact tags.

In addition, you may choose to use a custom tag instead of the default OwnerContact. In order to configure this, the mailer.yaml must be modified to include the contact_tags and the custom tag. The resource-owner will now email the custom tag instead of OwnerContact.

  - "custom_tag"

For reference purposes, the JSON Schema of the notify action:

  "type": "object",
  "required": ["type", "transport", "to"],
  "properties": {
    "type": {"enum": ["notify"]},
    "to": {"type": "array", "items": {"type": "string"}},
    "owner_absent_contact": {"type": "array", "items": {"type": "string"}},
    "subject": {"type": "string"},
    "priority_header": {"type": "string"},
    "template": {"type": "string"},
    "transport": {
      "type": "object",
      "required": ["type", "queue"],
      "properties": {
        "queue": {"type": "string"},
        "region": {"type": "string"},
        "type": {"enum": ["sqs"]}

Using on Azure


The mailer supports an Azure Storage Queue transport and SendGrid delivery on Azure. Configuration for this scenario requires only minor changes from AWS deployments.

You will need to grant Storage Queue Data Contributor role on the Queue for the identity mailer is running under.

The notify action in your policy will reflect transport type asq with the URL to an Azure Storage Queue. For example:

  - name: azure-notify
    resource: azure.resourcegroup
    description: send a message to a mailer instance
      - type: notify
        template: default
        priority_header: '2'
        subject: Hello from C7N Mailer
          - you@youremail.com
          type: asq
          queue: https://storageaccount.queue.core.windows.net/queuename

In your mailer configuration, you’ll need to provide your SendGrid API key as well as prefix your queue URL with asq:// to let mailer know what type of queue it is:

queue_url: asq://storageaccount.queue.core.windows.net/queuename
from_address: you@youremail.com
sendgrid_api_key: SENDGRID_API_KEY

The mailer will transmit all messages found on the queue on each execution, and will retry sending 3 times in the event of a failure calling SendGrid. After the retries the queue message will be discarded.

In addition, SendGrid delivery on Azure supports using resource tags to send emails. For example, in the to field:

  - tag:OwnerEmail

This will find the email address associated with the resource’s OwnerEmail tag, and send an email to the specified address. If no tag is found, or the associated email address is invalid, no email will be sent.

Deploying Azure Functions

The --update-lambda CLI option will also deploy Azure Functions if you have an Azure mailer configuration.

c7n-mailer --config mailer.yml --update-lambda

where a simple mailer.yml using Consumption functions may look like:

queue_url: asq://storage.queue.core.windows.net/custodian
from_address: foo@mail.com
sendgrid_api_key: <key>
    name: 'testmailer1'

Configuring Function Identity

You can configure the service principal used for api calls made by the mailer azure function by specifying an identity configuration under function properties. Mailer supports User Assigned Identities, System Managed Identities, defaulting to an embedding of the cli user’s service principals credentials.

When specifying a user assigned identity, unlike in a custodian function policy where simply providing an name is sufficient, the uuid/id and client id of the identity must be provided. You can retrieve this information on the cli using the az identity list.

    type: UserAssigned
    id: "/subscriptions/333fd504-7f11-2270-88c8-7325a27f7222/resourcegroups/c7n/providers/Microsoft.ManagedIdentity/userAssignedIdentities/mailer"
    client_id: "b9cb06fa-dfb8-4342-add3-aab5acb2abbc"

A system managed identity can also be used, and the Azure platform will create an identity when the function is provisoned, however the function’s identity then needs to be retrieved and mapped to rbac permissions post provisioning, this user management activity must be performed manually.

    type: SystemAssigned

Using on GCP


The mailer supports GCP Pubsub transports and SMTP/Email delivery, as well as Datadog and Splunk. Configuration for this scenario requires only minor changes from AWS deployments.

The notify action in your policy will reflect transport type projects with the URL to a GCP Pub/Sub Topic. For example:

  - name: gcp-notify
    resource: gcp.compute
    description: example policy
      - type: notify
        template: default
        priority_header: '2'
        subject: Hello from C7N Mailer
          - you@youremail.com
          type: pubsub
          topic: projects/myproject/topics/mytopic

In your mailer configuration, you’ll need to provide your SMTP account information as well as your topic subscription path in the queue_url variable. Please note that the subscription you specify should be subscribed to the topic you assign in your policies’ notify action for GCP resources.

queue_url: projects/myproject/subscriptions/mysubscription
from_address: you@youremail.com
# c7n-mailer currently requires a role be present, even if it's empty
role: ""

smtp_server: my.smtp.add.ress
smtp_port: 25
smtp_ssl: true
smtp_username: smtpuser
  type: gcp.secretmanager
  secret: projects/12345678912/secrets/smtppassword

The mailer will transmit all messages found on the queue on each execution using SMTP/Email delivery.

Deploying GCP Functions

GCP Cloud Functions for c7n-mailer are currently not supported.

Writing an email template

Templates are authored in jinja2. Drop a file with the .j2 extension into the a templates directory, and send a pull request to this repo. You can then reference it in the notify action as the template variable by file name minus extension. Templates ending with .html.j2 are sent as HTML-formatted emails, all others are sent as plain text.

You can use -t or --templates cli argument to pass custom folder with your templates.

The following variables are available when rendering templates:

variable value
recipient email address
resources list of resources that matched the policy filters
event for CWE-push-based lambda policies, the event that triggered
action notify action that generated this SQS message
policy policy that triggered this notify action
account short name of the aws account
region region the policy was executing in
execution_start The time policy started executing

The following extra global functions are available:

signature behavior
format_struct(struct) pretty print a json structure
resource_tag(resource, key) retrieve a tag value from a resource or return an empty string, aliased as get_resource_tag_value
format_resource(resource, resource_type) renders a one line summary of a resource
date_time_format(utc_str, tz_str='US/Eastern', format='%Y %b %d %H:%M %Z') customize rendering of an utc datetime string
search(expression, value) jmespath search value using expression
yaml_safe(value) yaml dumper

The following extra jinja filters are available:

filter behavior
utc_string|date_time_format(tz_str='US/Pacific', format='%Y %b %d %H:%M %Z') pretty format the date / time
30|get_date_time_delta Convert a time delta like '30' days in the future, to a datetime string. You can also use negative values for the past.

Developer Install (OS X El Capitan)

Clone the repository:

$ git clone https://github.com/cloud-custodian/cloud-custodian

Install dependencies (with virtualenv):

$ virtualenv c7n_mailer
$ source c7n_mailer/bin/activate
$ cd tools/c7n_mailer
$ pip install -r requirements.txt

Install the extensions:

python setup.py develop

Testing Templates and Recipients

A c7n-mailer-replay entrypoint is provided to assist in testing email notifications and templates. This script operates on an actual SQS message from cloud-custodian itself, which you can either retrieve from the SQS queue or replicate locally. By default it expects the message file to be base64-encoded, gzipped JSON, just like c7n sends to SQS. With the -p | --plain argument, it will expect the message file to contain plain JSON.

c7n-mailer-replay has three main modes of operation:

  • With no additional arguments, it will render the template specified by the policy the message is for, and actually send mail from the local machine as c7n-mailer would. This only works with SES, not SMTP.

  • With the -T | --template-print argument, it will log the email addresses that would receive mail, and print the rendered message body template to STDOUT.

  • With the -d | --dry-run argument, it will print the actual email body (including headers) that would be sent, for each message that would be sent, to STDOUT.

Testing Templates for Azure

The c7n-mailer-replay entrypoint can be used to test templates for Azure with either of the arguments:

  • -T | --template-print

  • -d | --dry-run

Running c7n-mailer-replay without either of these arguments will throw an error as it will attempt to authorize with AWS.

The following is an example for retrieving a sample message to test against templates:

  • Run a policy with the notify action, providing the name of the template to test, to populate the queue.

  • Using the azure cli, save the message locally:

$ az storage message get --queue-name <queuename> --account-name <storageaccountname> --query '[].content' > test_message.gz
  • The example message can be provided to c7n-mailer-replay by running:

$ c7n-mailer-replay test_message.gz -T --config mailer.yml