What’s the difference between 5.1.1 and 5.2.2?

5.1.1 and 5.2.2 indicate different failure types:

5.1.1: Bad destination mailbox address:

The mailbox does not exist. User never existed or was deleted. Immediately suppress this address. Classic "user not found" error.

5.2.2: Mailbox full:

The mailbox exists but has no storage space. User may clear space and become reachable. May retry or treat as soft bounce initially. Convert to hard after persistent failures.

Key differences:

5.1.1: Address is invalid. Remove immediately. 5.2.2: Address valid but inaccessible. May recover.

Handling implications:

5.1.1 warrants immediate suppression. 5.2.2 may allow limited retry before suppression.

5.1.1 is "nobody home." 5.2.2 is "home but not accepting deliveries."