I was configuring a main smarthost all my other machines should relay though, all Postfix. I configured everything as I should, including allowing Postfix to use PLAIN auth (important step, smtp_sasl_security_options =). However, it still didn’t work.

cannot authenticate to server relay.mx.mydomain[123.123.123.169]: no mechanism available

If you forget smtp_sasl_security_options =, you would get the following error:

cannot authenticate to server relay.mx.mydomain[123.123.123.169]: no worthy mechanisms available

Note the small but important difference: worthy. All Google hits on this problem tell you to allow PLAIN if you get the “worthy” error, except I already did, and this was not the error. #postfix couldn’t help either 🙁 Dug somewhat further…

The missing key? The libsasl2-modules package is also needed, not only libsasl2 (restart Postfix after installing). Tada.