zmailer local delivery gets stalled

[Felix Lee <flee@guardian.cse.psu.edu>]

User runs /usr/ucb/mail, and suspends the process while it's busy
rewriting the mailbox.  The user's mailbox is now locked indefinitely.
If the user gets new mail, zmailer's mailbox delivery agent will block
indefinitely waiting to get a lock.

A couple of these, and you've hit the maxchannel limit for local
delivery.  Local delivery is now stalled indefinitely.

So, either maxchannel for local delivery should be something very
large, or "mailbox" should abort if it can't acquire a lock
immediately.

Does this make sense?
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