How often are you polling for mail? If you poll too often, you don't give
your internet connection enough time to contact your ISP mail server and
download before potentially having to try to do it all again. Especially if
you're AV software is scanning your incoming and outgoing mail. If you have
multiple profiles that you're pulling mail for, this could exacerbate the
problem.
If you're polling more frequently than 10 minutes, please increase it and
see if that resolves the issue. If not, reconfigure your AV software to not
scan your incoming and outgoing mail. This won't compromise your security
since your real time scanner will still be active.