Rather than having Postfix deliver mail directly, you can configure Postfix to send mail via another mail relay (smarthost). Examples include [Mailgun](https://www.mailgun.com/), [Sendgrid](https://sendgrid.com/) and [AWS SES](https://aws.amazon.com/ses/).
Depending on the domain of the sender, you may want to send via a different relay, or authenticate in a different way.
Setting these environment variables will cause mail for all sender domains to be routed via the specified host, authenticating with the user/password combination.
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.
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`.
!!! note
You still have to define `RELAY_HOST` to activate the feature
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:
Thanks to the author of [this article][1] for the inspiration. This is also worth reading to understand a bit more about how to set up Mailgun to work with this.