Is this an IMAP account? Which destination folder is designated for sent messages in Account Settings > account name > Copies & Folders > When sending messages automatically place a copy in ... ? If it is an IMAP account you also may R-click on the account name > Subscribe .... and then in the Folder List check another folder such as "Sent" or "OUTBOX" to be displayed in the folder pane. Sometimes it's trial & error.. ...