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Standard SMTP codes · success reply

2.6.4: delivered after a lossy conversion

Usually a warning on a successful delivery: the message arrived, but a conversion on the way lost some of its content.

Standard
Provider
Success (2xx)
Class
Sources checked
By , email deliverability consultant
Author

At a glance

Code
2.6.4
Temporary or permanent
SuccessThe server accepted the command or the message.
Who has to act
The sender. The fix is on the sending side: your domain, your server or your list.
What the server says
2.6.4 Conversion with loss performed

What it means

X.6.4 is normally reported as 2.6.4, a success with a caveat: the message was delivered, but a conversion along the way, such as a format change at a gateway or headers re-encoded, could not keep everything. RFC 4468 shows it as 250 2.6.4 for MIME header conversion with loss. RFC 3463 adds that it may also be a permanent error, 5.6.4, if the sender has said that conversion with loss is prohibited; in that case the message is returned instead of being delivered changed.

Common causes

  • A gateway that changed the format of the message and dropped parts it could not carry.
  • Non-ASCII headers re-encoded by a server that could not keep them exactly.

How to fix it

  1. 01No action is needed for delivery; check with the recipient that the content looks right.
  2. 02Use standard encodings for headers and attachments to avoid conversions on the way.
  3. 03If it repeats with one recipient, ask their administrator which gateway converts the mail.

Sources

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

  1. 5.6.3Conversion 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.StandardPermanent
  2. 5.6.5Conversion failed.A server tried to convert the message content so it could be delivered, and the conversion did not succeed.StandardPermanent

All 105 standard codesEvery code in the wikiPaste your own bounce

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