5.6.3: conversion needed but not possible
The content has to be converted before it can be passed on, but a server on the path is not able to perform that conversion.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.6.3
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
5.6.3 Conversion required but not supported
What it means
RFC 3463's example for X.6.3 is a gateway that accepts 8-bit mail but cannot downgrade it to 7-bit for a next hop that requires it. RFC 4468 shows 554 5.6.3 for binary content that its server cannot convert to base64. In both cases the sender can avoid the problem by sending content that needs no conversion. The registry pairs X.6.3 with reply 554 and the condition does not change between attempts, so 5.6.3 is the form you meet, and the message is returned.
Common causes
- Raw 8-bit or binary content sent through a server that cannot downgrade it for the next hop.
- Content in a form the gateway cannot translate for the destination system.
How to fix it
- 01Encode every part with base64 or quoted-printable before sending.
- 02Update sending software that sends raw 8-bit content without checking the server's 8BITMIME support.
- 03Read the reply text for the part that could not be converted.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.6.2Conversion needed but not allowed.The content would have to be converted to be delivered, but the message itself or the sending host's policy forbids that conversion.
- 5.6.1Content type not supported.The message uses a kind of content that the delivery protocol or the next system on the path cannot carry, so it was refused.
- 5.6.5Conversion failed.A server tried to convert the message content so it could be delivered, and the conversion did not succeed.
- 554Transaction failed.The server permanently refused the transaction, often after receiving the message data or straight away when the connection opened.
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