Well it depends.
If you want to send mail through plan smtp your are likely going to need the assistance of the ISP of that particular network.
Most networks dont allow you to send stuff from port 25 and if that works its likely gonna be considered spam.
The majority of ISP's Ive seen have a specific address like smtp.thisisp.com, so depending on the provider for that specific proxy you need a way to obtain this data.
Smacking it through google like that basically blows all the need for any proxy in general.
Apart from the fact that google (like all major companies) keep very good track of what is a proxy and what not, its big business.
I personally would take a low level approach in combination with a module such as this:
http://socksipy.sourceforge.net/Routing it to the 'local' smtp server to me seems like the best approach.
I would suggest reading about that module ^ and find a clever way of using it for your purpose.
(maybe there are native proxy solutions/modules for sending email , that would be the easy way
)