what mail provider? What anti virus? Does turning off email scanning help?

Thanks for the reply Matt. Iam using windows defender for fire wall and anti virus. Turning these off makes no difference. I am using win 8.1 and have kept all updates current. I have tried deleting the user files and forcing thunderbird to rebuild them all to no avail.

But what mail provider? I have a similar problem with Hotmail, (in one of it's many names), because their system does not work fast enough.

When Thunderbird sends the delete command Outlook.com moves the mail to a folder called POP in Deleted. But it is still there a quarter of an hour latter when I check again and get a duplicate. You would think 15 minutes would be enough even if they had a kid on a bicycle moving things around. But apparently not.