3-digit reply code Temporary

SMTP error 455

Server unable to accommodate parameters.

Updated Jul 1, 2026

The short answer

Server unable to accommodate parameters. Common cause: A valid command carried parameters the server cannot handle right now (RFC 5321).

Quick fix

Retry, or drop the unsupported ESMTP parameter.

4 - Temporary. Safe to retry. The message is not lost and most servers retry automatically.

How to fix it

Retry, or drop the unsupported ESMTP parameter.

Related codes

420 TCP read/write timeout — non-standard, mostly GroupWise. 447 Delivery time expired — a misread of 4.4.7, not a real code. 449 Exchange routing error — non-standard, legacy Exchange only. 458 Unable to queue messages for node — the ETRN failure reply. 471 Local filter or Exchange block — not a valid SMTP code. 422 Mailbox full — a misread of enhanced status 4.2.2, not a real code.
All SMTP error codes
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