When I copy all my emails from my old account onto a local folder using the copy add on, majority of the emails fail and only some are actually copied, and I get a message saying that this many emails have failed to copy. When I try to copy the failed emails again none of them are copied onto the local folder. I cant move the old emails into my new account without saving them on a local folder first.

If anyone has a sure-fire way of migrating two accounts other than this please let me know