Introduction

Rather than having Postfix deliver mail directly, you can configure Postfix to send mail via another mail relay (smarthost). Examples include Mailgun, Sendgrid and AWS SES.

Depending on the domain of the sender, you may want to send via a different relay, or authenticate in a different way.

Basic Configuration

Basic configuration is done via environment variables:

RELAY_HOST default host to relay mail through, empty will disable this feature
RELAY_PORT port on default relay, defaults to port 25
RELAY_USER username for the default relay
RELAY_PASSWORD password for the default user

Setting these environment variables will cause mail for all sender domains to be routed via the specified host, authenticating with the user/password combination.

Note for users of the previous AWS_SES_* variables: please update your configuration to use these new variables, no other configuration is required.

Advanced Configuration

Sender-dependent Authentication

Sender dependent authentication is done in config/postfix-sasl-password.cf. You can create this file manually, or use

setup.sh relay add-auth <domain> <username> [<password>]

An example configuration file looks like this:

@domain1.com           relay_user_1:password_1
@domain2.com           relay_user_2:password_2

If there is no other configuration, this will cause Postfix to deliver email throught the relay specified in RELAY_HOST env variable, authenticating as relay_user_1 when sent from domain1.com and authenticating as relay_user_2 when sending from domain2.com.

NOTE to activate the configuration you must either restart the container, or you can also trigger an update by modifying a mail account.

Sender-dependent Relay Host

Sender dependent relay hosts are configured in config/postfix-relaymap.cf. You can create this file manually, or use

setup.sh relay add-domain <domain> <host> [<port>]

An example configuration file looks like this:

@domain1.com        [relay1.org]:587
@domain2.com        [relay2.org]:2525

Combined with the previous configuration in config/postfix-sasl-password.cf, this will cause Postfix to deliver mail sent from domain1.com via relay1.org:587, authenticating as relay_user_1, and mail sent from domain2.com via relay2.org:2525 authenticating as relay_user_2.

Excluding Sender Domains

If you want mail sent from some domains to be delivered directly, you can exclude them from being delivered via the default relay by adding them to config/postfix-relaymap.cf with no destination. You can also do this via

setup.sh relay exclude-domain <domain>

Extending the configuration file from above:

@domain1.com        [relay1.org]:587
@domain2.com        [relay2.org]:2525
@domain3.com

This will cause email sent from domain3.com to be delivered directly.

References

Thanks to the author of this article for the inspiration. This is also worth reading to understand a bit more about how to set up Mailgun to work with this.

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