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5.6.5 bounce: content conversion failed

5.6.5Conversion failedPermanent: retrying will not help

Typical wording
554 5.6.5 Conversion failed

What it means

X.6.5 was added in RFC 3463 as “conversion failed”: a server attempted a content conversion, which was allowed and supported, but the attempt did not work. The message was therefore not delivered in either its original or its converted form.

Failures usually come from malformed input. A charset label that does not match the bytes, base64 with invalid characters, or a nested MIME structure the converter cannot walk will all make it give up. Exchange and other groupware servers also raise conversion errors when turning MIME into their internal format.

Some servers send 4.6.5 when the failure might be a temporary resource problem, and the queue will try again. A 5.6.5 is final until the message itself is fixed.

Likely causes

  • Text declared as one charset (for example ISO-8859-1) but actually encoded in UTF-8, or the reverse.
  • Corrupt base64 or quoted-printable parts produced by a buggy generator.
  • Deeply nested or very large MIME trees that exceed the converter’s limits.
  • TNEF (winmail.dat) or calendar parts the receiving converter cannot parse.

How to fix it

  1. Save the source of the bounced message and validate its MIME structure; look for mismatched charset labels and broken encoded parts.
  2. Declare UTF-8 for all text parts and make sure the bytes really are UTF-8.
  3. In Outlook or Exchange, set messages to internet recipients to use MIME rather than TNEF (Rich Text) to avoid winmail.dat attachments.
  4. Simplify the message (fewer nested parts, smaller inline images) and send it again.

Checks that apply

  • Email Header Analyzer Shows the declared Content-Type, charset and transfer encodings, which is where most conversion failures start.
  • MailFlow Monitor Lets you confirm a fixed template now arrives end to end without a conversion bounce.
See the example diagnosed

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

FAQ

How is 5.6.5 different from 5.6.3?
With 5.6.3 the server could not convert at all. With 5.6.5 it tried and the conversion broke, usually because the input was malformed.
Does this mean the message was damaged in transit?
Rarely. It nearly always reflects how the message was built at the source, so fix the generator rather than the network.

All 191 bounce codes