SMTP Error 455

SMTP reply code 455 is a standard, RFC-defined response. RFC 5321 §4.2.3 lists it as "Server unable to accommodate parameters", and §4.1.1.11 defines exactly when it is sent: if the server is temporarily unable to accommodate one or more parameters associated with a MAIL FROM: or RCPT TO: command, and no more specific code applies, it returns 455. The permanent counterpart is 555 ("MAIL FROM/RCPT TO parameters not recognized or not implemented").

What causes SMTP Error 455?

The "455" is attached to a MAIL/RCPT line that carried an ESMTP parameter the server won't accept right now. Because 455 is a 4yz transient negative completion reply (RFC 5321 §4.2.1), the command was rejected but the condition is expected to clear, so the same message may be retried unchanged. Crucially, 455 is the generic transient parameter error: §4.1.1.11 says it is used only when the parameter's own definition does not mandate a more specific code. Common triggers:

• An ESMTP extension parameter the server advertised but can't process transiently, where no more specific code is mandated for that condition. The server falls back to the generic 455.
• Rate limiting / greylisting / load. Many receivers overload 455 to mean rate limiting or system load — i.e. "slow down and retry" — even though the RFC framing is about a parameter.
• A genuinely malformed or unexpected parameter — though a clearly invalid parameter is more often a permanent 501 ("syntax error in parameters") or 555, not 455.

Note that some extensions define their own codes rather than using 455. For example, the ESMTP SIZE extension (RFC 1870) declares message size on MAIL FROM and specifies 452 ("insufficient system storage") when the server temporarily can't accept a message that large, and the permanent 552 ("message size exceeds fixed maximum message size") when it exceeds the hard limit — so a size problem typically surfaces as 452/552, not 455.

Note also: the older version of this page said 455 "occurs on the recipient's side, there is nothing you can do." That is misleading. It is the receiver that emits 455, but the trigger is a parameter your sending side put on the command, and the correct sender behavior is to retry, not to give up.

How do I fix SMTP Error 455?

1. Treat it as transient — queue and retry with backoff. A standards-compliant MTA (Postfix, Exim) will keep the message queued and retry automatically. Libraries like Nodemailer or Python smtplib instead surface the 4xx error to your application (e.g. as a responseCode/SMTPResponseException) — your code (or an underlying queue) needs to catch it and retry with backoff.
2. Read the full reply text and any enhanced status code (e.g. 4.x.x). The 3-digit 455 is generic; the trailing text usually names the offending parameter or says "rate limit"/"try again later."
3. Trim or drop the problematic parameter. If the text names an ESMTP extension your sender adds optionally, retry without it. (If the text instead points at message size, you'll usually see a 452/552 rather than 455 — send a smaller message or split attachments.)
4. Lower send rate / batch size. If 455 correlates with bursts, throttle connections and recipients per transaction; reconnect after delays.
5. If it persists for hours, the condition may not be transient after all. Check whether the server is actually returning the permanent 555 (parameters not recognized or not implemented), which means you must stop sending that parameter rather than retry. Contact the receiving server's postmaster if the reply text is unclear.