5.5.5: protocol version mismatch
The two mail systems could not agree on a version of the delivery protocol, and they could not work around the difference on their own.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.5.5
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
5.5.5 Wrong protocol version
What it means
RFC 3463 defines X.5.5 for a protocol version mismatch that the two sides could not resolve automatically. SMTP itself has little version negotiation, so the code is rare; it is more likely at a gateway between different mail protocols, or with very old or unusual software. RFC 3463 ties it to no class: 5.5.5 means the message was returned, while a 4.5.5 means the condition might clear. When it appears, the sending software or a gateway on the path is the place to look.
Common causes
- A gateway translating between mail protocols whose versions do not match.
- Very old or non-standard mail software on one side.
How to fix it
- 01Read the reply text for the versions involved.
- 02Update the sending software, or relay through a standard mail server.
- 03Ask the recipient's administrator which protocol their gateway expects.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.5.1Invalid SMTP command.The sending software issued an SMTP command that the server does not support, or sent it at the wrong point in the conversation.
- 5.3.3Requested feature not supported.The message relies on a feature that the receiving system cannot provide, typically at a gateway that cannot carry it into another mail environment.
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