master: Warning: /mnt/foo is no longer mounted. See http://wiki2.dovecot.org/Mountpoints
Dovecot wants to keep track of mountpoints that might contain emails. If such a mountpoint is suddenly not mounted, it's handled as an error condition rather than as "this is a new user". This prevents other confusion that follows such a situation.
The list of mount points is maintained in the file /var/run/dovecot/mounts.
If you permanently removed a mountpoint, you can tell Dovecot to forget about it with:
doveadm mount remove /mnt/foo
Dovecot already internally filters out many mountpoints and filesystems that are pretty much guaranteed not to contain any emails, but of course it can't know about everything. This is especially problematic with filesystems that are sometimes mounted and sometimes not. You can permanently tell Dovecot to ignore a mountpoint with:
doveadm mount add /mnt/foo ignore
or ignore all mountpoints under /mnt:
doveadm mount add '/mnt/*' ignore
Mountpoints are added automatically when Dovecot starts up. If you mount a new filesystem with mails while Dovecot is running you can also add it manually:
doveadm mount add /mnt/foo