I've tried installing versions 3.0.3, 3.1, and now 3.1.2 and am running into the same problem. Over VPN, I use Lotus Notes R5 for E-Mail and without VPN, I use Outlook 2002. In both cases, when I try to send mail, the connection to the mail server times out without sending my note. The size of the note doesn't seem to be an issue.
As soon as I uninstall the VPN Client, I am able to send mail via Outlook without problems. I'm still trying to get my hands on V3.5 of the client but, based on other notes I'm seeing here, others have got VPN and XP working happily.
I've tried playing with MTU but haven't had any success yet.
Anyone have any ideas? Is there a registry setting that might be an issue?