A bounce needs a status code, not a mood
Answer When a solo ad "bounces," ask for the status code. RFC 3463, from January 2003, makes the first digit do the real work. A 4 is a temporary failure. A 5 is permanent. Code 5.1.1 means the mailbox address is bad. A mood is not a count, and a count without a class is not a reason to pay again or to refund.
The note that started this: RFC 3463: Enhanced Mail System Status Codes, IETF, January 2003.
Sellers and buyers talk about bounces as if the word were a measurement. It is a drawer. Inside the drawer are delays, full mailboxes, policy refusals, and addresses that were never real. If you mix them, you will suppress people who were only busy, or you will keep mailing people who were never there. Both mistakes cost the domain.
The note that started this is RFC 3463, Enhanced Mail System Status Codes. It is old infrastructure, which is why it still shows up in the reports a serious platform can export. The status is three numbers. The first is the class: 2 for success, 4 for a persistent transient failure, 5 for a permanent failure. The second number says what the failure is about, such as the address or the mailbox system. The third narrows it. You do not need the whole catalog to buy a solo ad. You need the class, and you need 5.1.1 recognized on sight.
What is the difference between a 4 and a 5 bounce?
In RFC 3463 the first digit is the class. A 4 is a persistent temporary failure. A 5 is a permanent failure. They are not the same event.
A 4 says "not now." The receiving system may be busy, the mailbox may be over quota, or the message may be delayed for a reason that can clear. Retry logic exists because 4 exists. Treating every 4 as a dead subscriber shrinks a good list for no reason. Treating every 4 as a success hides a send that did not finish.
A 5 says "stop." Continuing to hammer a permanent failure is how a domain teaches mailbox providers that it does not listen. On a solo ad, a high permanent failure rate is a fact about the file, not about your swipe. If you sold the send, that fact is yours to own before you take the order. If you bought it, that fact belongs in the refund conversation, next to whatever the quote said about list quality.
What does status 5.1.1 mean?
RFC 3463 defines 5.1.1 as a bad destination mailbox address. The mailbox named in the address does not exist at that system.
The domain might be real. The local part is not a mailbox there. This is the classic dead address: a typo, a closed account, a role address that was never a person, a record someone bought instead of earned. It is not a complaint. It is not an open. It is the system saying the envelope was addressed to nobody.
Ask the seller what share of the file came back 5.1.1 on recent mail of their own, before your guest email goes out. You are not owed their whole suppression list. You are owed a straight answer about whether the file is alive. Best solo ad vendors have that answer because they mail often enough to see the codes. A seller who has not mailed the file since spring is guessing.
What should you ask when a seller says the solo ad bounced?
Ask for the enhanced status code and the count of addresses in each class, and compare a permanent failure to the refund line in the quote.
Three numbers will do. How many addresses were attempted. How many came back class 5. How many came back class 4 and are still retrying. If they only have the word bounced, their platform is hiding the only useful column, or they have not looked. Either way you are negotiating in the dark.
Solo ad pricing that ignores permanent failures is a price for a file on paper. Put a threshold in the quote if you are going to argue about it later. Then keep the export. The 2003 standard already gave you the vocabulary. Use it the day the seller says the mail bounced, and do not let the conversation stay at the level of a mood.