Bounce codes are technical messages, but the business question behind them is usually simple:
Should this address stay in future sends, or not?
The broad split
Most bounce codes fall into one of two buckets:
- 5xx style failures, which are usually more permanent
- 4xx style failures, which are usually more temporary
That split matters more than memorizing every individual number.
Common permanent-style codes
These usually point to:
- nonexistent mailboxes
- invalid address formats
- domains or recipients that cannot accept the message
The practical response is usually suppression or removal.
Common temporary-style codes
These often point to:
- server availability issues
- temporary mailbox problems
- throttling or short-term infrastructure limits
The practical response is usually retry and monitor, not immediate removal.
Why list cleaning still matters
The codes show you what already happened. Cleaning helps prevent the worst of the permanent failures before the send.
That is why MailCull fits upstream of the bounce report. You want fewer obvious failures reaching the campaign in the first place.