I have just recently received the following message multiple times when sending to an e-mail address that I have used often.  Any idea what the problem is and how I can resolve it?

Message:

This message was created automatically by the mail system (ecelerity).

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

>>> *** Email address is removed for privacy *** (while not connected): 554 5.4.7 [internal] exceeded max time without delivery

Thank you!