You can usually receive mail fro one ISP while connected to another, but not send. Mail servers are usually restricted to only allow mail to be sent out form their own network, otherwise they are used by spammers.
I don't know about Freeserve broadband, but when I used their, they redirected all port 25 requests to their own server, so it didn't matter what you set up as your SMTP server.
"Insanity: doing the same thing over and over again and expecting different results." (Albert Einstein)