The three limits that end an undelivered message verifiedimportantWritten by the Hub

Seven retry intervals covering four hours and ten minutes; a hundred attempts; a deadline of a day. Whichever comes first wins, and on the shipped configuration the retry list ends long before the other two. What happens then is the part the package does not state. Either the last interval repeats until the deadline — giving roughly ten more attempts at two-hour spacing — or attempts stop and the message waits out its day before expiring. The difference is the whole retry behaviour of a node after the first four hours, which is where most temporarily deferred mail lives. For Hub this is not academic: a message expiring at the deadline is counted as expired, not as refused, so an unexplained gap in the retry schedule would show up in analytics as expiry rather than as a delivery problem. The open question below is worth answering before anyone tunes these numbers for a customer.