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5.6.2 bounce: conversion required and prohibited

5.6.2Conversion required and prohibitedPermanent: retrying will not help

Typical wording
554 5.6.2 Conversion required and prohibited

What it means

X.6.2 is the RFC 3463 code for “conversion required and prohibited”. A server along the path could deliver the message only by converting it (for example from 8-bit to 7-bit transfer encoding, or from one character set to another), but conversion was not allowed.

Conversion is normally prohibited for one of two reasons. The message may be signed, and changing it would break the signature (DKIM or S/MIME). Or the sender asked for no conversion, for instance by a Content-Conversion header or through a gateway policy that preserves messages exactly.

This code is uncommon on the modern internet because almost every server supports 8BITMIME. When it appears, it usually points at an old relay or a specialised gateway such as a fax, X.400 or archive bridge on the recipient’s side.

Likely causes

  • Sending an 8-bit body to a hop that lacks 8BITMIME, when that hop refuses to downgrade the message.
  • A signed message routed through a gateway that would need to re-encode it, and refuses because that would invalidate the signature.
  • A legacy gateway (X.400, fax or pager) that cannot convert MIME parts and has conversion disabled.
  • A message header that explicitly prohibits content conversion.

How to fix it

  1. Encode outgoing text as quoted-printable or base64 so the message is already 7-bit clean and needs no conversion on any hop.
  2. For signed messages, sign after applying 7-bit transfer encoding, so later relays have nothing to change.
  3. Remove any header that forbids conversion unless you truly depend on it.
  4. If the failing hop belongs to the recipient, send them the bounce so their admin can enable 8BITMIME or allow conversion on that gateway.

Checks that apply

  • SMTP Diagnostics Shows whether the receiving MX advertises 8BITMIME, which tells you if your 8-bit body can travel unconverted.
  • Email Header Analyzer Reveals the Content-Transfer-Encoding and any conversion headers on your message, so you know whether it is 7-bit safe.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

How do I know which hop refused?
The Reporting-MTA and Remote-MTA fields in the delivery status notification name the server. If it is not the first MX, the problem is inside the recipient’s network.
Is 5.6.2 related to 5.6.3?
Yes. 5.6.2 means conversion was possible but forbidden, while 5.6.3 means it was needed and the server simply could not do it.

All 191 bounce codes