I had this question after viewing Exchange Message Reject 5.4.7
I've been told by my customer that they are getting this NDR: #< #5.0.0 smtp; 5.4.7 - Delivery expired (message too old) 'EOF' (delivery attempts: 0)> #SMTP#
when sending email to one of the accepted email domain in my Exchange server.
What does it means and where does the email being hold up before it is expired and discarded?
There is another SmartHost (Virtual Appliance) VM that is running email hygiene check, but so far I did not find any error in the spam filter.