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SMTP 555: MAIL FROM or RCPT TO parameter rejected

555MAIL FROM/RCPT TO parameters not recognized or not implementedPermanent: retrying will not help

Typical wording
555 5.5.4 Unsupported option: SMTPUTF8

What it means

RFC 5321 defines 555 as "MAIL FROM/RCPT TO parameters not recognized or not implemented". Extended SMTP lets a client add parameters after the address, such as `SIZE=1048576`, `BODY=8BITMIME`, `SMTPUTF8`, `RET=HDRS`, or `NOTIFY=FAILURE`. If the server has not advertised the matching extension, it answers 555.

This usually happens when a client assumes features without reading the EHLO reply, or when it falls back to HELO (which advertises nothing) and still sends parameters. The enhanced code is typically 5.5.4, "Invalid command arguments".

It is permanent for that command; the client must resend without the parameter. Compare with 455, which says the parameter is known but cannot be met right now, and 504, which is about parameters of other commands such as AUTH.

Likely causes

  • The client sends `SMTPUTF8` or a non ASCII address to a server that does not support RFC 6531.
  • DSN parameters (NOTIFY, RET, ENVID, ORCPT) are sent to a server without the DSN extension.
  • The client fell back to HELO after an EHLO failure but kept sending ESMTP parameters.
  • An intermediate proxy removes extensions from EHLO while the client still uses them.

How to fix it

  1. Compare the parameters in the failing MAIL FROM or RCPT TO line with the extensions the server listed after EHLO.
  2. Configure the client or library to send only advertised parameters; most libraries do this when they parse EHLO properly.
  3. For SMTPUTF8 failures, use an ASCII address for that recipient or route through a server that supports internationalised mail.
  4. If a proxy is involved, bypass it and see whether the error disappears.

Checks that apply

  • SMTP Diagnostics Prints the server’s EHLO extensions so you can see whether SIZE, DSN, 8BITMIME, and SMTPUTF8 are really offered.
See the example diagnosed

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FAQ

Will removing the parameter lose anything?
Only the feature it asked for, such as a delivery status notification or the size hint. The message itself can still be delivered.
Why does 555 happen only for some recipients?
Different recipient domains run different servers; one may support SMTPUTF8 or DSN and another may not.

All 191 bounce codes