- In general, we should be NFS-safe.

 - NFS server must support fcntl() locking currently. It's possible to
   add file-based locking code, but I haven't bothered.

 - Modifylog uses fcntl() for figuring out when to delete the log file, and
   assumes that changing file locking between F_RDLCK / F_WRLCK is atomic
   (not sure if this is the case with all operating systems, I hope so).
   This anyway could be more difficult to change not to use fcntl().

 - gethostname() must return different name for each IMAP server accessing
   a user's mailboxes

 - Clocks should be somewhat synchronized:
    - maildir: One minute difference is enough to have a small chance of
      new mail to get temporarily lost until more mail arrives.
    - One hour is enough to create a problem when two imap servers try to
      open the same mailbox at the same time.