- 252 response code, defined
- 252 is the SMTP reply meaning the server cannot verify the recipient but will accept the message and attempt delivery, which is a deliberate refusal to confirm anything.
The code exists in the specification for the VRFY command and gets reused at RCPT TO by servers that would rather not expose their user list. Yahoo is the best-known example, and the behavior is intentional.
The reasoning is sound from their side. A server that answers honestly hands an attacker a free way to enumerate every valid account by trying addresses until one is accepted.
What you get in practice is a message that will be taken, then bounced later if nobody is there. The failure moves from the handshake to an asynchronous report hours afterwards.
For verification this is the single hardest response to handle with integrity, because it is indistinguishable in usefulness from silence. There is no further probe that resolves it.
How ZapBounce reports it
A 252 returns unknown, never valid, and carries the reason cannot_verify. It is not billed. Folding this code into the valid bucket is the specific choice that lets other vendors publish a 99% figure.
The reply on the wire, and what follows it
The code's original wording in the SMTP standard is: 252 Cannot VRFY user, but will accept message and attempt delivery. It was written for the VRFY command, which asks a server directly whether a user exists. Servers that answer a recipient command this way are borrowing the same meaning.
Suppose a probe asks about lee@ at a domain with this policy and gets a 252. Ask about a made-up neighbor and you get 252 again. There's no contrast between a real address and a fake one, so the probe has learned nothing.
If you go on to send a real message, it's accepted. When there's no such mailbox, the server generates a bounce notice afterwards and mails it to your return path. That can take minutes or hours. Your sending platform has to catch that notice and tie it back to the original message, a less dependable process than reading a refusal during the conversation.
Fourteen hundred addresses nobody can score
Put numbers on it. Say 1,400 addresses in your 10,000-row list sit at providers that answer every recipient with a 250 or 252 by policy. An honest verifier reports those as unknown, and the most coverage it can claim on the file is 86%.
A second verifier stamps all 1,400 valid. Its report shows full coverage and a much shorter list of problems. If 8% of those mailboxes are gone, you've been handed 112 hard bounces labeled as safe, and you paid for each of those verdicts.
Don't mix this up with 251, a neighboring code that means the user isn't local and the server will forward the message. A 251 is a real yes about a real recipient. It's also rare on the modern internet. When you evaluate a vendor, ask which verdict a 252 maps to and whether it's billed. At ZapBounce it comes back as unknown and isn't charged.
252 response code: common questions
Why would a server return 252?
To avoid confirming which accounts exist. Answering accurately would let anyone harvest a full user list one probe at a time.
Does a 252 mean my message will be delivered?
It means it will be accepted. If the mailbox does not exist, the bounce arrives later as an asynchronous report.
Which providers return 252?
Yahoo and AOL are the most common. Some corporate mail systems configure the same behavior deliberately.