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