What’s the difference between hard, soft, and transient bounces?
A hard bounce is a five xx SMTP response code for example 550 User unknown. It is a permanent failure also called a permerror and the address must be suppressed immediately.
A soft bounce is a four xx SMTP response code for example 452 Mailbox full. It is a temporary failure and may clear on retry.
Transient bounces are another name for soft bounces. The term emphasizes that the condition is temporary and the sending Mail Transfer Agent should retry delivery after a set interval.
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