As soon as the filter program has taken a hold on your message, you can rest
assured the mail will get filtered one way or the other. If filter can't
queue your mail, it will exit with an exit status of 75, that status
being recognized by "sendmail" as a "deliver later on" hint, in which case
the mail message will safely wait in sendmail's queue.

So if filter gets your message, it immediately forks and exits with a 0
status for sendmail, letting it know its work is finished and releasing it
to save resources. It then calls mailagent on the queued message (in mailagent's
private queue) to actually process the message.

Only after successful processing will mailagent delete the queued message.
Hence, under an heavily loaded system, the worst that could happen would be
a duplicate processing of a message, or a bounce back when sendmail cannot
fork and exec the filter program from your .forward.

Under catastrophic conditions, filter or mailagent will simply dump the message
on stdout, for ~/.bak to catch, preceded by the reason why processing was
aborted.