Message Not Delivered: Message expired before delivery completed
The message could not be delivered because the message was considered too old by the receiving system. It may have been delayed while being processed.
Fixes
Double check the recipient's email address to make sure it is correct and resend the message.
If that does not work, consider contacting the recipient(s) through an alternative email address or other means.
5.4.7: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.4.7:
The message was considered too old by the rejecting system, either because it remained on that host too long or because the time-to-live value specified by the sender of the message was exceeded. If possible, the code for the actual problem found when delivery was attempted should be returned rather than this code.