Apple Mail generating Mail[378]: No worthy mechs found. What is it? I know it's Apple mail because the PID of the running Mail is 378

378 ??        44:44.93 /Applications/Mail.app/Contents/MacOS/Mail -psn_0_28679


In the Syslog file I'm getting is:

Mail[378]: No worthy mechs found


Does anyone have any idea why this shows, or what this is.

I'm running postfix and dovecot on 10.10.3
I just solved this problem (on a system that was NOT running postfix) by changing the authentication method on one email account from Password to MD5 Challenge-Response.