>On top of that it's virtually guaranteed that your ISP explicitly
>forbids running services on your home Internet connection, and
>probably even mentions email as a service you're not allowed to
>run. Most large ISPs also block all TCP/25 traffic going through their
>networks that is not aimed at their own email servers (which is why
>TCP/587 is so popular for SMTP submission with third party email
>providers), and you HAVE to use that port for server to server email
The reason ISPs block TCP/25 is _not_ part of their 'no email service of your own' policy. Blocking of TCP/25 stems from the early days of spamming, when spammers would first relay through, and later hijack consumer PCs for spamming. This was often combined with relaying through a company's mail server, but even when companies got wise to this and changed their setup to not relay, the home PCs could still continue sending spam. Blocking outgoing port 25 put a stop to this (and many companies also block their outgoing port 25, except for their mail server, simply because if an internal computer got infected they don't want to get a spew of spam coming from their network. Just like how the ISPs are thinking). Port 587 requires authentication and in practice only allows you to connect to mail servers you're known to, and there's no reason to block this so the ISPs don't.
As another poster mentioned below: If you show that you know what you're doing, i.e. that there's little risk your computer will be one of the infected spam-forwarding PCs, you can often get your ISP to remove the TCP/25 block.