Issue: Delayed messages with error "451 Failed in remote data - psmtp"
Symptoms: Messages are delayed with a 451 error. Eventually the message is either successfully received by the addressee after a delay, or the message is returned to the sender after a few days because it was in the queue for too long.
Resolution: The message security service throws this deferral when the sending MTA* has completed transferring the message data, the security service has processed the message and has decided to forward it on to the receiving MTA*, and then, while it was in the process of transferring the message, the connection closed abruptly or the session was ended prematurely. Because of this abrupt ending of the session between the security service and the receiving MTA*, this deferral is issued back to the sending MTA* so that it will retry the message at a later time.
It is often difficult to determine for certain why a session between the security service and the receiving MTA* ended prematurely. However, it usually happens just once due to a timeout, which can occur for any number of reasons. When the sending MTA* retries, the message gets through.
In cases where the message is repeatedly retried and fails consistently, it indicates that there is something preventing the message from transferring from the security service to the receiving MTA*. Checking the logs on the receiving MTA* may have information that would explain the failures.
In the absence of any clear cause for the premature ending of the session, there is a possible work around for this problem. Adding the sender to their whitelist or to the Blocked Senders list causes the security service to accept the message from the sending MTA* immediately rather than having to transfer it to the receiving MTA*.
*MTA - Short for message transfer agent, the program responsible for receiving incoming e-mails and delivering the messages to individual users. The MTA transfers messages between computers. Hidden from the average user, it is responsible for routing messages to their proper destinations. MTAs receive messages from both MUAs and other MTAs, although single-user machines more often retrieve mail messages using POP. The MTA is commonly referred to as the mail server program. Microsoft Exchange Server is an example of a popular MTA.
*MUA - Short for mail user agent, the software that allows a user to access and manage e-mail, including reading, composing, disposing, printing and displaying e-mail messages. The MUA provides the interface between the user and the MTA. Thunderbird and Outlook are examples of MUAs.