I was running with pop3_uidl_format = %f, and well, that doesn't work with mdbox very well, and when a user ran a UIDL command, it errored the user out.
Simple fix, but would probably be nicer to log a config error when started dovecot instead.
I converted all my box's over to mdbox, from maildir and ran into this
issue, I knew exactly what it was and fixed it, but might want to log
a config error or something for others to notice the issue.
