On a Windows Server 2012 with Exchange 2013 installed, you might have a problem with emails getting stuck in the submission queue, and after 2 days you get an NDR message:
#550 4.4.7 QUEUE.Expired; message expired ##
I had this problem with meeting invites that contains an html signature and on some e-mails.
The problem is related to Exchange 2013 when it is installed on Windows 2012.
Workaround is not to use RTF formatted emails, and don’t use my signature in meeting invites.
I have also changed the transport service so messages will expire after 1 hour, and notifying the users that the RTF formatted email could not be delivered.
You can change the transport service with this command:
Set-TransportService “SERVERNAME” -MessageExpirationTimeout 01:00:00
The problem should be fixed in a future update for Exchange 2013.