Well-configured other servers will just dutifully keep trying to deliver messages. There are plenty of less well configured servers out there. For instance, I

Well-configured other servers will just dutifully keep trying to deliver messages. There are plenty of less well configured servers out there. For instance, I have greylisting configured on my primary SMTP server, but no greylisting on my secondary (because it's run by a friend, not me). Some servers will contact mine, offer a message, get told "try again", and just not bother, even though they could try the secondary and get through straight away. Other servers will give up after failing to deliver for 30 minutes.