Today a number of packages were available on my 10.1 production box, one of them being smeserver-qpsmtpd-2.7.0-12.
One of my users that sends and retrieves mail using thunderbird over S/POP and Secure SMTP noted that he could retrieve email, but sending produced an incorrect password error message.
After tracking logs etc and changing the password to something completely different things worked again. Setting the password from web admin to what it previously was made the issue re-appear.
Thinking that this might have something to do with the characters in the password, I removed on a hunch its last character which was double-quotes ("). After doing so, the password was happily accepted when trying to send email.
Opened issue 12395 for it:
https://bugs.koozali.org/show_bug.cgi?id=12395