Cannot append message to Sent folder

Is there a way to remove the offending message from the "outbox" to avoid the attempts to APPEND the message to the sent folder?

Yes, if you can locate the message then you can hold down ⌥ and use “Message ▸ Delete Now”, but then you loose the message. If you just delete the message then it is moved to “Deleted Messages” where the problem would likely reoccur. If you have another account then you could also try to move the message to that account.

Do you have any idea why the server rejects it? It's not the size of the message itself since it is only 45711 bytes.

(Naturally the server should report something more meaningful, but that is unlikely to ever change.)

I managed to delete the message file and avoid the APPEND issue, sadly I didn't keep a copy for further investigation. If it recurs I'll check it out further - so we can close this.

Thanks