[Gordonjcp]

That's basically what domestic ISPs do. You will probably find that outbound traffic on port 25 is blocked, because all of your pwn3d inadequately-patched Windows machines are spam cannons now.

[bombcar]

Yep - some block it so hard you have to use other ports to communicate with offsite mail servers (and why various other ports are found, now).

Some ISPs will remove the block if you ask.