When you set-up the e-mail address, the pop server will be the respective service providers, however, when you set the SMTP server, both addresses will use the ACTUAL service providers server.
For example, you use Freeserve and your partner uses Onetel.
Your server addresses will be;
Your Partners will be;
mail.onetel.net.uk (pop server)
smtp.freeserve.net (smtp server)
In other words, you can only SEND mail via the service provider you are connected to, but you can RECEIVE from anywhere.