I’m running a server that I recently upgraded to Centos 7. For a few months, it wouldn’t send mail and today I finally decided to do something about it. I was getting an error AUTH warning: no mechanisms. After messing around for a while, I saw this in /etc/mail/access:
# If you want to use AuthInfo with "M:PLAIN LOGIN", make sure to have the # cyrus-sasl-plain package installed.
Once I installed the cyrus-sasl-plain package and restarted saslauthd and sendmail, I could send mail (over a TLS link only) again.