Just thinking there should be some sort of control mechanism to END this endless loop.
The below steps you through how to produce this issue.
USE IT AT YOUR OWN RISK.
Setup a domain and add 2 mailboxes to it.
Be sure it is a proper domain with proper DNS MX records pointed at the SM server.
Make sure you have message archiving turned on for the domain.
Record your current domain level settings for this domain for the inbound message delivery settings and the status of the deliver locally if exists switch (so you can switch them back later).
Now at the domain level in SM SHUT OFF the deliver locally if exists switch, and
set the inbound message delivery to "External (use MX record)".
Now send an email from one of the accounts you created to the other.
The messages are delayed in appearing in the SMTP logs, but they will appear.
They also appear in the Message Archive.
From what I can see there is nothing that STOPS this at any point it just keeps going and going.
I am seeing about 5 delivery attempts per minute.
So nothing jaw dropping, but probably not a good idea to have a run-away process like
this in the server.
During this looping...
I NEVER got a response back to the sending mailbox (would expect a bounce).
And the message never arrives at the destination mailbox.
Once satisfied that this truly is an issue, you can STOP it by setting the deliver Locally if exists and the inbound message delivery back to the settings they were originally (I told you to record them... didn't listen huh?)... and click the save. The message at this point will finally deliver to its destination.
BUT... all the backlogged attempts will attempt to deliver after that as well. It seems SM cleans them out as duplicates??? Not sure.