When you send an e-mail, you receive an error message showing a 4.4.7 error message: Non-Delivery message
RESOLUTION:
A 4.4.7 NDR message usually indicates a problem
on the receiving server. Check the validity of the recipient's
address, as well as whether the receiving server is configured to
receive messages correctly. If you then resend the message, it is
placed in the queue again, and if the receiving server is up, the
message is delivered
A 4.4.7 NDR message can also indicate that a message header limit has been reached on a remote server or that some other protocol timeout has occurred while communicating with the remote server. It may be necessary to reduce the number of recipients in the header of the message for the host to which you are receiving this error. If you then resend the message, it is placed in the queue again, and if the receiving server is up, the message is delivered.
An example of the typical bounce-back NDR message is shown below:
-----Original Message-----
From: System Administrator
Sent: DateTime
To: 'Recipients_Name'
Subject: Undeliverable: Subject_Of_Original_Message
Your message did not reach some or all of the intended recipients.
Subject: Subject_Of_Original_Message
Sent: DateTime
The following recipient(s) could not be reached:
'Recipients_Name' on 3DateTime
Could not deliver the message in the time limit specified. Please
retry or contact your administrator.
<FQDN_Of_Sending_Server#4.4.7>