5.5.5 bounce: wrong protocol version
5.5.5Wrong protocol versionPermanent: retrying will not help
500 5.5.5 Wrong protocol version
What it means
X.5.5 is “wrong protocol version” in RFC 3463. It says the peers do not share a version of the mail protocol, so the transaction cannot go ahead. It is one of the rarer codes you will meet.
In practice it shows up when one side expects LMTP (RFC 2033, used between an MTA and a local delivery agent) and the other speaks SMTP, or when a client tries ESMTP features against a very old server that only knows HELO. Some appliances also use it when a client sends EHLO to a service expecting LHLO.
The problem is almost always a configuration mismatch inside one organisation, between its own MTA and its own mailbox store. External senders rarely cause it, which is why a 5.5.5 in a bounce is usually a job for the recipient’s admin.
Likely causes
- An MTA configured to hand mail to a mailbox server over SMTP when the mailbox server listens with LMTP, or the reverse.
- A transport map or smarthost entry pointing at the wrong port, for example the LMTP port 24 instead of 25.
- An outdated server that does not understand EHLO and answers with a protocol version error instead of falling back.
- A proxy or load balancer that speaks a different protocol on its back end than on its front end.
How to fix it
- If you are the sender, forward the bounce to the recipient’s postmaster; the mismatch is inside their mail path.
- If you run the receiving system, check the transport between the MTA and the mailbox store. In Postfix, `mailbox_transport = lmtp:unix:private/dovecot-lmtp` must point at a socket that really speaks LMTP.
- Confirm which protocol each listener expects by connecting to it and comparing the greeting with the command you send (EHLO for SMTP, LHLO for LMTP).
- Restart the services after correcting the transport, then send a test message and watch the logs for a clean handoff.
Checks that apply
- SMTP Diagnostics Shows whether the public MX greets you as a normal ESMTP server, which narrows the fault to the internal hop behind it.
- TCP Port Test Checks which ports answer on a host, useful when you suspect an LMTP or SMTP listener on the wrong port.
FAQ
- Can I fix 5.5.5 from the sending side?
- Normally not. The mismatch sits between the recipient’s own servers, so their administrator has to correct the transport.
- Is my message lost?
- It bounced back to you, so nothing was delivered. Send it again once the recipient confirms the fix.