OT: Need some proxy advice

This is not much related to SBS but I've gotten so much great advice
here, I had to give it a shot.

I have a problem that I hope an email proxy can take care of but I
need some advice

We are moving our small office and have 4 users using OE that connects
to pop3 from our cable provider (optonline.net)

The new office is verizon fios and I will eventually switch our email
accounts (actually going to Exchange on SBS), but for the move I just
want to keep our email the same (optonline.net)

I have VPN routers at both locations and have managed to create a site
to site vpn that connects both networks together but the problem is
that when trying to send email from the verizon network using the
optonline.net accounts that cablevision blocks port 25 from a non
optonline network (no relay I believe).

The router at the new office routes internet traffice out through
verizon and only 10. traffic across the VPN so I'm stuck in this

Is it possible to put a windows based proxy on a computer at the old
office that the OE clients at the new office will send and receive
mail through which will then send it out to the optonline.net smtp
gateway and allow me to keep everyone running until I get our new
server running with exchange and have an orderly switch to a new
domain name?

winproxy or something similar? I've just started looking into this
but it seems like it would work.

Need to know if it's possible, and suggestions would be great as to
what product I could use. Thanks and I appreciate the input.