VPN and PPPoE

Has anyone run into this problem?
We have set up a VPN and users on Cable modem can log on from home move around the network, send/receive e-mail with out problem.
A user on DSL (PPPoE) can connect and move around the network with no problem andalso receive e-mail. But when he tries to send an e-mail it locks up and wont send. We have narrowed it down to either the fact that its PPPoE or something in his router.
We have messed with MTU Speeds and everything and still cannot figure this little bug out.
He is on a Windows XP Laptop connected to a wireless LAN over a wireless Linxsys router.
If anyone can help out with this one it would be greatly appreciated.

Thanks in advance!

Paul