SMTP error 5.5.2 Syntax error

What SMTP error 5.5.2 means

The server could not parse the line it received. Not the wrong command at the wrong moment, which is 5.5.1, but text it could not interpret at all: a malformed greeting, an address with stray characters, an unrecognised verb. It typically arrives as 501 5.5.2 with the offending line echoed back. Registered only as a permanent failure, so the sender does not retry, and a system sending this way never succeeds until the malformation is fixed.

Mail delivery protocol status
Sent by The receiving server, unable to make sense of what it was sent
Occasional

What SMTP error 5.5.2 Syntax error actually means

Think of 5.5.1 as the server saying it understood you and will not comply, and this as the server saying it could not read you. The most frequent real trigger is the opening greeting. SMTP expects a client to introduce itself with a fully qualified domain name, or with a bracketed address literal, and a device announcing itself with its short internal name or as localhost is refused outright by strict receivers. Because the failure is permanent and deterministic, it is oddly easy to diagnose: the same message fails at the same point every time, and the reply text usually contains the exact string the server objected to.

A mail transaction protocol command was issued which could not be interpreted, either because the syntax was wrong or the command is unrecognized. This is useful only as a permanent error.
— RFC 3463

How 5.5.2 appears in a bounce

A server reporting this condition sends it alongside a three-digit reply code, and a bounce prints the two together. The registry lists these pairings for this status: 500 5.5.2, 501 5.5.2, 502 5.5.2, 550 5.5.2, 555 5.5.2. Which one you get depends on the point in the conversation at which the server refused.

The leading 5 is the server's verdict rather than part of the code's identity: it marks this as a permanent failure, which means the sending server has given up and returned the message.

What causes SMTP error 5.5.2 on a business phone system

A greeting using a short or invalid hostnameAppliances frequently introduce themselves with their NetBIOS name, their model number or localhost. A receiver that insists on a proper domain name refuses the session before any message is offered.
Addresses containing spaces, commas or unbalanced bracketsRecipient lists pulled from a spreadsheet or a database carry whatever was typed into them, including trailing commas, quotation marks and display names that were never separated from the address. The server rejects the line rather than guessing.
Non-ASCII characters where the session cannot support themAn accented character or a smart quotation mark pasted into an address, on a connection with no support for international addressing, produces a line the receiver cannot interpret.
Incorrect line endings from hand-written codeSMTP requires a carriage return and line feed together. Scripts that write to a socket directly often send a line feed alone, which works against tolerant servers and fails against strict ones. The result is an application that delivers to some domains and not others.
A firewall corrupting the conversation as it inspects itProtocol inspection that rewrites commands can leave them subtly malformed by the time they arrive. The sending application and the receiving server are both behaving correctly and the device between them is not.

How to fix SMTP error 5.5.2

Capture the exact line that was rejectedServers usually quote the offending text back in the reply. That string is the whole diagnosis, and it is worth getting from a protocol log rather than from a summary in an application's error screen.
Give the sending device a real fully qualified nameSet the greeting name to a hostname that resolves publicly, and ideally one that matches the reverse lookup on the address you send from. That single change fixes a large share of these rejections and improves deliverability generally.
Validate addresses where they are entered, not where they are sentTrim whitespace, reject anything containing a space or a comma, and separate display names from addresses in the database. Catching a malformed address at the point of entry is far cheaper than tracing it back from a bounce weeks later.
Correct line endings in bespoke sending codeUse an established mail library rather than writing to the socket by hand. Libraries handle line endings, line length limits and encoding correctly, which is most of what this code is complaining about.
Disable protocol inspection and retest before blaming the applicationIf the same code succeeds from outside the network and fails from inside it, the firewall is altering the session. Turn the inspection off, prove the difference, then leave it off.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about SMTP error 5.5.2

Will the message be retried?

No. This is registered as permanent, so the sending system gives up on the first attempt and returns the message. That is deliberate: a line the receiver cannot parse will be equally unreadable in an hour, so retrying would achieve nothing.

Our application sends fine to most places and fails at a few. How can the message be malformed?

Because servers differ in how strictly they enforce the rules. A tolerant receiver silently repairs a short greeting name or an odd line ending, while a strict one refuses it. The message is malformed in both cases; only one of them tells you.

Does this mean the recipient address is invalid?

Not necessarily. A syntactically invalid destination address is reported as 5.1.3 specifically. This code is about any part of the conversation the server could not parse, which is just as often the greeting or the sender as it is the recipient.

Related SMTP status codes

Source. SMTP error 5.5.2 Syntax error is defined in RFC 3463 and registered in the IANA SMTP Enhanced Status Codes registry. The causes and fixes above are drawn from our own experience supporting UK business email systems.

All Email delivery & SMTP codes