Associated question: using an EXITCODE=77 denotes an "insufficient
permission" on bounced mail. Would not an EXISTCODE=69 ("service
not available") be more useful? Is there really a difference in
the final scheme of things?
Haven't checked with the sendmail docs, but, logically, I think
that such a message would be requeued for later sending (sendmail
OT3d, or O Timeout.queue* in v8.8).