outbound queue is filling up with NDRs
outbound queue shows "421 4.7.7 [internal] Unable to process message at this time".
Log shows "A message from < > source <internal> to <[email protected]> using sender's group policy <unknown> returned Disposition: <none>." and "Error while attempting to resolve sender address: ."
AD has an entry with empty email address like "smtp:" which ends up matching the empty sender of the NDR
delete the empty entry in AD
SMG, Address resolution enabled and outbound rules active