When an email gets the SMTP error Relaying Denied, can AMS be configured to give up re-trying to send the email and send a failure message to the sender.
In a future update we do plan to greatly change how the outgoing SMTP works. Basically as it is now, AMS continues to try regardless of the error. This behaviour is of course not preferable for some. Although we do not yet have a date for this, we will also be adding support in the same update for things such as ETRN, a new queuing system and more.
Something similar is also in the new design (still no date yet), basically we would be allowing you to create bands of time in which mails in the queue would pass through the longer they are in the queue. These bands (you control the timing and how many) would control how the mail is delivered, which would include how often to retry, the route to take...etc.