Have a look at the last posts in the thread "Beta version 1.1b1 available".
I believe you're experiencing the PPTP MTU problem.
(BTW my setup is identical to yours)
When retrieving mails via pop3 the packet size reaches MTU (I've just
checked with Ethereal).
While a permanent solution is (hopefully) coming up, you can change Windows
PPP-MTU for now - if you are using Windows 2K, XP or 2003.
Set the MTU-value in this Registry key:
Set this value to 1396 or less - That should fix packet loss when using PPTP
from inside to WAN.