5.0.0: unspecified failure

What it means

X.0.0 is the catch-all of the enhanced status codes. The server knows only whether the problem is permanent or temporary, and gives no subject or detail. With 5.0.0 the message has been refused and comes back to the sender; with 4.0.0 the condition is expected to pass, so the sending server keeps the message in its queue and tries again. RFC 3464 names these generic codes as the fallback when nothing more precise fits, so 5.0.0 often appears when a gateway translates an error from another system or when software does not map its errors to specific codes. The reply code before it and the text after it usually carry the real reason.

Common causes

• The receiving software has no specific code for its error and falls back on the generic one.
• A gateway translated an error from another mail system and could not map it to a precise code.
• A policy or content rejection where the operator chose not to reveal the reason.

How to fix it

1. 01Read the whole bounce: the reply code and the text after the code normally name the actual problem.
2. 02For 4.0.0, let the sending server retry, and act only if the message stays queued or comes back.
3. 03Look up the reply code (550, 554, 451) on its own page for its likely meanings.
4. 04If the text gives nothing, ask the recipient to have their administrator check the receiving server's logs.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• RFC 3463, section 3.1: Other or Undefined Statusrfc-editor.org
• RFC 3464, Appendix B: Guidelines for gatewaying DSNsrfc-editor.org
• IANA, SMTP Enhanced Status Codes registry (Enumerated Status Codes)iana.org

Further reading

• SMTP error codes explained
• Soft bounce vs hard bounce