My recollection is that Office365 stores a copy in sent folder, regardless. I believe you can UNtick the saving of the sent message and it will still be there.

Yes I saw that in another post so I tried it and there was no email in the Sent folder.

Is the same sent being used on the IMAP Server account and Thunderbird. Check in account settings > copies and folders If it is set to "Sent" it is almost certainly wrong, as that is not the folder name office uses. Therefore the issue may be the folder specified simply does not exist bubbling up as a failure to copy. As the correct sent folder is supposed to be automatically negotiated from the servers CAPA command it is generally not an issue. Bit There are always exceptions.

My recollection is office folder name is "sent items" modify the setting to use the second entry where you get a drop down to set the sent folder to be used.

Free live accounts certainly make a copy of the email as it goes through the outgoing server and place it in the server sent folder. I assume office would do the same as both are basically the same infrastructure. Please check using a web browser that there is no copy in the server sent folder.

We are all assuming your account is IMAP because of the type of error, but it would be good if you could confirm that.