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SMTP 455: server unable to accommodate parameters

455Server unable to accommodate parametersTemporary: the sender retries

Typical wording
455 4.5.4 Server unable to accommodate parameters

What it means

RFC 5321 lists 455 as "Server unable to accommodate parameters". It answers MAIL FROM or RCPT TO when the client added ESMTP parameters (such as SIZE, BODY, RET, or NOTIFY) that the server recognises and supports in principle, but cannot satisfy at this moment.

It is the temporary twin of 555. With 555 the server does not recognise or implement the parameter at all, so retrying with the same command will fail again. With 455 the parameter is known and the condition might change, for example once disk space frees up for a large declared SIZE.

In daily operations 455 is rare. Most servers answer resource problems with 452, so if you see 455 it usually comes from a specific MTA or appliance that follows the RFC wording closely.

Likely causes

  • The declared SIZE in MAIL FROM is larger than the free spool space the receiver currently has.
  • A DSN option such as NOTIFY or RET cannot be honoured because the receiver’s DSN subsystem is temporarily unavailable.
  • A gateway or security appliance in front of the mail server cannot pass a parameter through for now.
  • A client sends an extension parameter after the server stopped advertising it during a reload or failover.

How to fix it

  1. Capture the full SMTP dialogue (MAIL FROM line with its parameters and the 455 reply) from your MTA log.
  2. Let the queue retry; the server may accept the same parameters once its condition changes.
  3. If it repeats, compare the parameters you send with the extensions the server advertises after EHLO, and drop any it no longer lists.
  4. If you run the receiving side, check free space against message size limits and the health of any DSN or filtering component.

Checks that apply

  • SMTP Diagnostics Lists the extensions the server advertises after EHLO (SIZE, DSN, 8BITMIME, SMTPUTF8), which tells you which parameters it should accept.
  • MX Lookup Identifies which MX host you actually talked to, useful when only one of several hosts answers 455.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

How is 455 different from 555?
455 is temporary: the parameter is understood but cannot be met right now. 555 is permanent: the parameter is not recognised or not implemented.
Does my mail server retry a 455?
Yes. Like other 4xx replies, it leaves the message queued and tries again later.

All 191 bounce codes