This bug can only be fixed (at all, not to mention "quickly") if we are able to get additional info from you.
- Whether this happens with all messages or just some
- Is there a pattern (as to which messages are affected)
- Whether this happens with any mail app on the receiving side or only a specific app
- Finally, we'll need sample messages 1) what you forward 2) what gets sent, in .eml format (all headers and all text).
For sample messages, you can use a desktop mail app to "forward as .eml" (Outlook, Thunderbird both have this, if you one of those).
This is also a common feature in web mail - in particular, in Gmail you can click on the little arrow next to the reply icon, choose "show original" and "save" the original message from the new tab that opens.