How to use Postfix as Relay for Mailgun

Mailgun is a popular SMTP Relay/API service, one of my favorite. For transactional emails I have favored Mandrill before they declared to shutdown and later on merged with Mailchimp. Mandrill has cleaner network than any other services for transactional emails till this date. But what if, you need a smtp relay along with transactional emails? Mandrill fails there, as they can’t be entirely used as a SMTP relay. For those cases, I prefer Mailgun over Sendgrid, one of the main reason is, Sendgrid has poor network quality over Mailgun.

If you try to configure Sendgrid with Postfix, you will see, it will work without smtp_sasl_auth_enable set to true/yes. But this won’t be the case with Mailgun. To use Mailgun as smtp relay, you need to set the following in your main.cf file:

# set the relayhost to use 587 port of mailgun:
relayhost = [smtp.mailgun.org]:587

# set the authentication using static method:
smtp_sasl_password_maps = static:postmaster@yourdomain.com:password
# you can get the smtp authentication from Sending >> Domain Settings >> Select Domain >> SMTP Credentials

# set sasl authentication on
smtp_sasl_auth_enable = yes

# set sasl security options to no anonymous:
smtp_sasl_security_options = noanonymous

Once these are done, you can save the file and restart postfix to start relaying with Mailgun. In cases, if you see the following error:

SASL authentication failed; cannot authenticate to server smtp.mailgun.org[34.224.137.116]: no mechanism available

Along with the following:

warning: SASL authentication failure: No worthy mechs found

This means, you are lacking the SASL authentication library for postfix or libsasl2 is not enough to cover the dependencies. To resolve this, you can install the cyrus-sasl libraries. You may do that using the following:

yum install cyrus-sasl cyrus-sasl-lib cyrus-sasl-plain

This should be it, your SMTP should now send mails using Mailgun as the relay.

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