Have you ever wondered what all those SMTP status codes mean? It’s not that hard to read. Each code consists of three numbers. The format for this is class.subject.detail, for example 2.5.0 (also written as 250).
Here is a list of the classes:
2.x.x - Succes 4.x.x - Temporary failure 5.x.x - Permanent failure
Here are the subjects:
x.0.x - Other or undefined x.1.x - Addressing related x.2.x - Mailbox related x.3.x - Mail system related x.4.x - Network or routing related x.5.x - Mail delivery protocol related x.6.x - Message content related x.7.x - Security or policy related
Lastly, the detail:
x.1.0 - Other x.1.1 - Bad destination mailbox address (the part to the left of '@') x.1.2 - Bad destination system address (the part to the right of '@') x.1.3 - Bad destination mailbox syntax x.1.4 - Destination mailbox address is ambiguous x.1.5 - Destination address is valid x.1.6 - Destination mailbox has moved, no forwarding address x.1.7 - Bad sender's mailbox address (syntax) x.1.8 – Bad sender’s system address (the part to the right of ‘@’) x.2.0 – Other x.2.1 – Mailbox disabled, not accepting messages x.2.2 – Mailbox full x.2.3 – Per mailbox message length (i.e size) exceeded x.2.4 – Mailing list expansion problem x.3.0 – Other x.3.1 – Mail system storage full x.3.2 – System not accepting messages (destination down, excessive load, etc) x.3.3 – System not capable of selected features x.3.4 – Message too big for system x.3.5 – Configuration problem x.4.0 – Other x.4.1 – No answer from host x.4.2 – Bad connection x.4.3 – Directory server failure x.4.4 – Unable to route x.4.5 – Mail system congestion x.4.6 – Routing loop detected x.4.7 – Delivery time expired x.5.0 – Other x.5.1 – Invalid command x.5.2 – Syntax error x.5.3 – Too many recipients x.5.4 – Invalid command arguments x.5.5 – Wrong protocol version x.6.0 – Other x.6.1 – Media not supported (i.e. attachment) x.6.2 – Conversion required and prohibited x.6.3 – Conversion required but not supported x.6.4 – Conversion with loss performed x.6.5 – Conversion failed x.7.0 – Other x.7.1 – Delivery not authorised, message refused x.7.2 – Mailing list expansion prohibited x.7.3 – Security conversion required but not possible x.7.4 – Security features not supported x.7.5 – Cryptographic failure x.7.6 – Cryptographic algorithm not supported x.7.7 – Message integrity failure