When I click Send, a window pops up saying something like "Copying your message to the Sent folder" and a progress bar appears showing 100% completion. It remains on-screen for several seconds before disappearing. Sometimes, a copy appears in my Sent folder, sometimes TWO copies appear in my Sent folder and frequently, another window pops up saying there was a failure in copying my email to the Sent folder and asking to Retry. If I click Retry, another window pops up to say Copy Failed and no copy is made in my Sent folder. This occurs often and I have the latest of Thurderbird's multiple updates!