Am amazed this issue still going on, but not surprized that it hasn't been fixed. Am still onthe mailing list for any update on this matter, that's why I am replying.
After trying everything possible, including every thing listed in this forum and many others, and obviously apple, who privately admitted, that it is indeed a problem that many users have but they could not say when or if at all Cupertino would fix it or not.
Anyhow, what fixed mine was that I had my ISP completely erase/close my mail account and set up a new one on another server using exactly the same parameters/setups except for the passwords. And it works ever since !
Have to mention here, my mail account had been on that server for almost 6-7 years, so something will have changed somewhere, making my old server outdated.
Hope this helps someone, as I wish I had help then....