You are reviewing the MTA logs for a message that has been deferred. Which Delivery Status Notification (DSN) code indicates that the receiving…

Delivery Status Notification (DSN) codes, also known as Enhanced Mail System Status Codes, are defined by a three-digit structure (e.g., X.Y.Z). The first digit represents the status class. A 4.x.x class code signifies a "Persistent Transient Failure." This indicates a temporary issue with the receiving mail server or network path that prevented immediate delivery. The sending Mail Transfer Agent (MTA), such as Proofpoint, interprets this code as a signal to defer the message and retry delivery at a later time. This directly corresponds to a message being temporarily unable to be processed.

B. 2.x.x: This class indicates success. The message was successfully accepted and delivered by the recipient's mail server.

C. 3.x.x: This class is not used for SMTP delivery status codes as defined in the relevant standards (RFC 3463/5321).

D. 5.x.x: This class indicates a permanent failure or "hard bounce." The message was rejected, and the sending server will not attempt redelivery.

1. IETF RFC 3463

"Enhanced Mail System Status Codes":

Section 2

"Status Code Structure": This section defines the code classes. It explicitly states

"A status code of 4.XXX.XXX indicates a persistent transient failure... The sender is encouraged to try again later." This directly supports that 4.x.x codes are for temporary failures that result in deferral.

Available at: https://doi.org/10.17487/RFC3463

2. IETF RFC 5321

"Simple Mail Transfer Protocol":

Section 4.2.1

"Reply Code Severities and Theory": This document

which obsoletes RFC 2821

defines the primary SMTP reply codes. It specifies that 4yz replies are "Transient Negative Completion" replies

indicating a temporary failure where the command was not accepted

and the requested action did not occur

but the client is encouraged to try again.

3. Proofpoint Protection Server (PPS) 8.18 Log Reference:

Chapter 2

"Log Fields and Formats"

Section "SMTP Log Fields": In descriptions of the status field in MTA logs

the documentation explains that SMTP reply codes beginning with '4' (e.g.

451) are temporary failures that cause the Proofpoint MTA to queue the message for a later delivery attempt. This aligns with the industry standard and the scenario described.