I said this in another comment but this seems like a naive way to react to an "address does not exist error" that they've already delivered to before. The only legit scenario in which that happens is when the user deletes the address, which is a rare event (pretty much always <= 1 time in the lifetime of any address), and there shouldn't be anything wrong with treating that kind of situation the same as any soft error. If you're wrong, your mail will just get rejected a few more times anyway, and you'll know it's genuinely a dead end.
The underlying issue (wherever this occurs) seems to be lack of nuance regarding error codes when people try to implement robust systems. Different codes imply different things and shouldn't all just fall back into generic buckets.
The underlying issue (wherever this occurs) seems to be lack of nuance regarding error codes when people try to implement robust systems. Different codes imply different things and shouldn't all just fall back into generic buckets.