dovecot auth-worker error happens when I enabled the dovecot to consider both the system account and the MySQL virtual mailbox databases.

[Emmanuel Chanel]

I've found this for a long time. But I didn't examined "sudo apt-get 
libpam-smbpass" till now... Doing that, my problem is "solved"...
http://ubuntuforums.org/showthread.php?t=2246951&highlight=dovecot+auth+worker
libpam-smbpass has a bug of memory leak. I cannot tell surely....But it 
returns the error... So the auth-worker would get down by that.
Dovecot uses both my system accounts and MySQL virtual mailboxes now...

-- 
Emmanuel Chanel( emmanuelchanel at gmail.com )