5.5.2

Enhanced Status Code 5.5.2: Syntax Error

Permanent failure Medium severity Protocol RFC 3463
What it means

Enhanced Status Code 5.5.2 means “Syntax Error.” The server received a command it could not parse. Usually a malformed address, a stray character, or a line that broke the encoding the protocol expects.

At a glance
Code5.5.2
Bounce typeHard (permanent)
SeverityMedium
CategoryProtocol
What to doSuppress the address; do not retry
StandardRFC 3463
What it looks like in your mail logs 
501 5.5.2 Syntax error, cannot decode response

What does 5.5.2 mean?

Enhanced status code 5.5.2 means a syntax error. RFC 3463 defines it as a mail transaction protocol command that could not be interpreted. Where 5.5.1 means the command was understood but was wrong or unsupported, 5.5.2 means the server could not make sense of the characters at all.

Google returns it as 501 5.5.2 when it cannot decode a response, and as 555 5.5.2 for a syntax error that may close the connection. Malformed addresses are the usual trigger: an address with unquoted spaces, unbalanced angle brackets, a trailing comma from a badly joined recipient list, or a non-ASCII character in a context that has not negotiated SMTPUTF8. Bare line feeds instead of proper carriage-return line-feed pairs are the other classic, and they are invisible in most editors.

Because the fault is in the bytes rather than the intent, this is a code you can usually reproduce and fix quickly once you can see the raw session. It is also one that appears suddenly in systems that were fine for years, when a new data source starts supplying addresses that were never validated.

How 5.5.2 plays out

Your server attempts delivery
The recipient server returns a permanent 5.5.2 rejection
This is a hard bounce: the message will not be accepted as sent
Suppress the address and fix the root cause before resending

Where 5.5.2 sits: soft vs hard bounce

Soft bounce (4xx) Hard bounce (5xx)
NatureTemporaryPermanent
SMTP class4xx5xx
What to doLet it retrySuppress the address
Recoverable?OftenNo
5.5.2 is✓ this code

Common causes of 5.5.2

  • A recipient or sender address is malformed, with stray spaces, commas or brackets
  • Line endings use bare line feeds instead of carriage return followed by line feed
  • A non-ASCII character appears in a command without SMTPUTF8 being negotiated
  • A header value contains a control character or an unencoded newline
  • An address was concatenated incorrectly when building a recipient list
  • A command line exceeded the length the server accepts

How to fix 5.5.2

  • Validate and normalise every address before it reaches the SMTP conversation
  • Ensure all protocol lines terminate with carriage return followed by line feed
  • Encode non-ASCII header content properly, or negotiate SMTPUTF8 where supported
  • Strip control characters from any value that is interpolated into a header
  • Log the raw session bytes, since the offending characters are usually invisible in a log viewer
  • Check any newly added data source, since unvalidated imported addresses are a common trigger

Frequently asked questions

What does SMTP error 5.5.2 mean?
It means the server could not parse a command you sent. The characters did not form something it could interpret, which is different from 5.5.1 where the command was understood but was unsupported or arrived out of order. Malformed email addresses and incorrect line endings account for most cases.
Why does a valid-looking address cause a syntax error?
Because something invisible is attached to it. Trailing whitespace, a stray comma left over from splitting a list, an unbalanced angle bracket, or a zero-width character pasted in from a spreadsheet all produce a syntactically invalid address that looks perfectly normal on screen. Log the raw bytes rather than the rendered value.
What line endings does SMTP require?
Every line must end with a carriage return followed by a line feed. Bare line feeds are a frequent cause of syntax errors, particularly in code written on Unix systems where a single newline is the norm, and they are invisible in most editors. Check the raw bytes if you suspect this.
Can non-English characters cause 5.5.2?
Yes, if they appear in a protocol command without SMTPUTF8 having been negotiated. Internationalised addresses require the server to advertise SMTPUTF8 and the client to request it. Header content is different and should be MIME encoded rather than sent raw.
This started suddenly with no code change. Why?
Almost always a new data source. A fresh import, a new signup form without validation, or a partner feed supplying addresses in a different format will introduce malformed values into a system that has been stable for years. Validate at the point of entry rather than at the point of sending.

Related bounce codes

5.5.1 Invalid Command 5.5.0 Other or Undefined Protocol Status 501 Syntax Error in Parameters or Arguments 5.1.3 Bad Destination Mailbox Address Syntax 5.1.7 Bad Sender Mailbox Address Syntax

Sources & further reading

RFC 3463 · The standard that defines this reply code IANA registry · The authoritative list of enhanced status codes and their meanings Gmail SMTP errors and codes · Google Workspace reference for every code Gmail returns
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes