So if you are sitting at an ISP or a hotel and they have port 25
closed, you should be able to 'tunnel' through to your 'home' SMTP
server to send mail. Some sort of authentication would have to transpire
between your machine and your 'home' server so that spammers can't
hijack something like a 'username/password' combo.
I know that Microsoft in their AD functionality have lots of technology
that could be incorporated into something like this. (just a thought)
This is not an excuse for a lame SPF record. You can solve this with
SMTP AUTH and port 587 (I referencing memory here) or web based mail
or vpn connections etc. All solutions offer strong crypto for credentials
and data.
Rgds,
-GSH