Accounts with office365/hotmail/outlook.com or gmail have the property that they automatically copy sent messages to the IMAP Sent folder, so in TB you should uncheck 'Place a copy in' in the Copies & Folders section of Tools/Account Settings. Does that eliminate the error message on sending? Do the messages appear in the Sent folder when you access the account through webmail?

I have this problem too. Copying to sent folder does not happen. In the end have discard or just save but then can not retrieve message. I have Microsoft office 365 and use gmail and ymail.

I also have this problem ie sent emails not being copied into Sent folder. My Gmail account works fine, but I have the problem with my Plusnet accounts. What I have discovered with Thunderbird, is that if I set it to copy sent mail to a local folder rather than the Sent folder, it copies to the local folder fine. Bizzarly, if I then drag and drop the email from the local folder to the Sent folder it goes in with no problem and appears in the sent folder on all my devices and (obviously) on the Plusnet server. So I have found a solution albeit a somewhat tedious one. David

I have this problem some of the time using version 68.6.0 (64 bit). I have reply messages going to the folder of the original message, usually the inbox, in which case the error message reads " Your message was sent but a copy was not placed in your sent folder (Inbox) due to network or file access errors." This is with AOL mail (I got dumped there by Verizon).

Usually if I select "Save" I get the error message again, then if I select "Save" again the message just disappears.