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5.6.3 bounce: conversion required but not supported

5.6.3Conversion required but not supportedPermanent: retrying will not help

Typical wording
554 5.6.3 Conversion required but not supported

What it means

RFC 3463 calls X.6.3 “conversion required but not supported”. The message needs to change form before the next hop can take it, and the server holding it has no way to perform that change. Nothing forbade the conversion; the capability simply is not there.

Typical situations are binary MIME (BINARYMIME with CHUNKING) or 8-bit content travelling towards a server that only takes 7-bit data, through a relay that never implemented downgrading. Some gateways also use 5.6.3 when a recipient system requires a format (for example plain text only) that the relay cannot produce.

Unlike 5.6.2, there is no policy decision here. Fixing it means either sending in a format every hop understands or giving the relay the ability to convert.

Likely causes

  • Binary or 8-bit content sent through a relay that cannot re-encode it for a 7-bit-only next hop.
  • Use of the BINARYMIME extension end to end when one server on the path does not support it.
  • A gateway to a non-SMTP system (fax, pager or archive) that cannot transform the MIME parts it receives.
  • Old or minimal MTAs in embedded devices acting as relays.

How to fix it

  1. Configure your sending software to use 7-bit transfer encodings (quoted-printable or base64) for all parts.
  2. Turn off BINARYMIME on your side unless every server in the path supports it.
  3. If your own relay produced the code, upgrade it or route through a modern MTA that converts on the fly.
  4. Send a plain-text test message to confirm the path works, then re-send the original in a 7-bit-safe form.

Checks that apply

  • SMTP Diagnostics Lists 8BITMIME, CHUNKING and BINARYMIME support on the receiving MX, so you can see which formats it accepts directly.
  • MailFlow Monitor Proves end-to-end delivery of a test message after you change the encoding settings.
See the example diagnosed

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FAQ

Is the recipient’s server broken?
Not necessarily. It may be old or minimal. The robust fix is to send 7-bit-safe mail, which every server accepts.
Will my server retry a 5.6.3?
No. It is permanent, and retrying the same content would hit the same wall.

All 191 bounce codes