There are two "ends" to delivery delays:

1) client SMTP (Mail Transport Agent)

2) server SMTP (MTA)

The client is the one "closest" to the sender.  The server is the one
closest to the receiver.  Though it is possible that the server received
the email, that does not mean that it was delivered to the end user's
mailbox.  CCMAIL is fond of such receive-but-do-not-deliver.

Each of these MTA's and other internal processes can have their own
inherent delay timeouts.  Some are most annoying.  Your tolerance to pain
and your personal list-owner preferences will dictate how you deal with it.

/Pete Weiss at Penn State