Are you aware that all emails you send already have a copy of the original email in the Sent folder and creating bcc's is only creating a duplicate?

Are you aware that you can have copies of sent 'Replies' put in the same folder as the original recieved email. Right click on mail account in Fodler pane and selectt : 'Settings' select 'Copies & folders' select 'place replies in the folder of the message being replied to' click on OK

Is this an imap mail account ? Try going into offline mode and synchronising folders for offline use.

Once all in ready, in offline mode, try right click, select 'copy to' and choose the folder.

Thanks for your reply. Unfortunately, the 'sent' feature of Thunderbird only works about 40% of the time, that's why I created the 'copies' account. I do get the 'Replies' saved to my Inbox. This is a road runner account which i believe is pop3