ISP Bounce Classification

When an email bounce occurs, ISPs generally classify the bounce as being Hard or Soft. The main difference between the two is that a soft bounce may be resolved if resent again after a delay, whereas a hard bounce will most likely not succeed if resent. Technically, soft bounces are referred to as "Persistent Transient Failures", while hard bounces are referred to as "Permanent Failures". If you want to get to a higher level of technical detail, messages returned with a message status code of 5.xxx.xxx are hard bounces, while those with a 4.xxx.xxx code are soft bounces. The "x"s represent numbers which further specify the nature of the bounce and often vary widely based on a specific ISP's classification system. For the purpose of understanding email bounces within the application, you only need to know the first number in a message code.

So to bring it all home, message status codes beginning with a 4 represent a soft bounce, or "Persistent Transient Failure". Message status codes beginning with a 5 represent a hard bounce, or "Permanent Failure".