SMTP Error 455 - Server unable to accommodate parameters. | Warmup Inbox
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.
Most rejections come back to sender reputation.
Blocklists, auth failures, and policy blocks are what warmup prevents. Warm up your domain so your mail is trusted before you hit send.