I am testing out the new tunnel and have run into a couple of issues. I am now unable to send any email to my ISP's SMTP server whilst connected through your PPTP tunnel. I get a connection timeout. Also I am unable to connect to my internet facing router (I have a ADSL Netgear SG834G V4 router as primary router with a DLink DIR-655 as secondary router for gigabit networking. All traffic is passed as DMZ to the DLink). Oddly I can access the DLink fine but not the primary router when the tunnel is enabled. The main issue though is not being able to send any emails whilst connected to the tunnel.
Email over the ppp tunnels is blocked, using port 25. Change your port to 587.
I guess your isp is advertizing an ipv6 address for their smtp server, or else you wouldn't of had this issue.