Postfix as relay to a SMTP requiring authentication

Sometimes you may in need to use an external SMTP provider to send your emails, and usually ISPs give instruction on how to configure mail clients such as Outlook or Thunderbird. But what if you are already using an internal SMTP server such as Postfix?

These guidelines are for Debian (but may be helpful with other systems as well) and are related to Postfix. The SMTP provider in the example is AuthSMTP which is a well known provider for SMTP relaying.

Given you already have a working Postfix environment, first of all edit your main.cf and add these lines:

relayhost = [mail.authsmtp.com]
smtp_sasl_auth_enable=yes
smtp_sasl_password_maps=hash:/etc/postfix/sasl-passwords
smtp_sasl_mechanism_filter = digest-md5
smtp_sasl_security_options=

then, create with $EDITOR a file called /etc/postfix/sasl-passwords and fill it with something like this:

[mail.authsmtp.com] yourusername:yourpassword

then, compile the map file

# postmap hash:/etc/postfix/sasl-passwords

now we are almost done, just restart postfix and it should work.

Now, probably it won’t really work and you’ll start to see messages like these in your postfix log:

warning: SASL authentication failure: No worthy mechs found
SASL authentication failed; cannot authenticate to server mail.authsmtp.com

that’s because you are missing some SASL packages from Debian. Issue

# aptitude install libsasl2-modules

and it should install all the missing packages and make the thing work :)

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