Undeliverable email messages can have several causes. Here is a list of the most common errors you will experience:
"The Domain Name System (DNS) reported that the recipient's domain does not exist". This is telling us that the address somebody@somewhere.com is not valid. In specific, there is no email server at "somewhere.com"
"Your message to donotreply@somewhere.com couldn't be delivered. donotreply wasn't found at somwhere.com" This is telling us that there is a server at somewhere.com but there is no mailbox called donotreply
"When Office 365 tried to send your message, the receiving email server outside Office 365 reported an error". For this you need to scroll down to the bottom of the message to "Error Details" and check for the following messages:
"550 5.0.350 Remote server returned an error -> 552 1 Requested mail action aborted, mailbox not found" This tells us that the email address somebody@somewhere.com did not exist on the mail server at somewhere.com
"Delivery has failed to these recipients or groups: somebody@somewhere.com" Scroll down through the error messages and see if one of the following is listed:
"Remote server returned '550 5.4.300 Message expired -> 451 [Rl01] Message temporarily deferred" This indicates the message was never able to be delivered to the destination mail server. Above this there will be a listing of the retry attempts "Total retry attempts: 5" indicating that your mail server attempted to deliver it 5 times and was deferred every time. The two most common reasons for this are:
The destination mail server is too busy
The destination mail server is rejecting email from your email server
"Server at mta5.am0.somewhere.net (67.195.228.110) returned '421 4.4.42 Connection dropped due to ConnectionReset" This indicates that the communications were interrupted between your mail server and the destination mail server
"Server at mta6.am0.somewhere.net (98.136.96.75) returned '421 4.4.42 Connection dropped due to SocketError'" This indicates that the communications were interrupted between your mail server and the destination mail server
"Your message couldn't be delivered. Despite repeated attempts to connect the recipient's email system it didn't respond."
"Remote server returned '550 5.4.300 Message expired -> 451 [Rl01] Message temporarily deferred" This indicates the message was never able to be delivered to the destination mail server. Above this there will be a listing of the retry attempts "Total retry attempts: 5" indicating that your mail server attempted to deliver it 5 times and was deferred every time. The two most common reasons for this are:
The destination mail server is too busy
The destination mail server is rejecting email from your email server
If the recipients of the email messages are Yahoo or Google then additional DNS configurations are needed on the sending mail server. Please read these announcements from Yahoo and Google for detailed information about how to configure the sending mail server: