Dear Maintainer,

   On my system, home directories are automounted using autofs over NFS.
   It appears that dovecot starts before the autofs daemon is completely
   ready. Thus, it seems to be looking at the (empty) mount point
   and seeing no mailboxes in people's home directories.

   The workaround is to wait until the system is fully up, log in,
   and restart dovecot.

   It's taken me a while to work out what's going on; it seems to have come
   when I started using systemd instead of sysV init.  I suspect it's an
   ordering issue, but it's not that obvious how to make dovecot delay
   starting until autofs is ready to mount directories.