Help with Mail.app

I am using mail.app and for some reason one of my messages wants to use smtp.mac.com to send the message. Unfortuantly i dont have that service. If i try to select another one it won't allow me to. I have earthlink and it works fine, i just sent an e-mail out but this one won't go.

The message is from a rafting place and the message wants a reply to confirm the appointment but it wants to use the stmp.mac.com instead of my earthlink one.

How do i get it to use my earthlink outgoing mail server??

Thanks

have you tried redirecting it to yourself, so you'd reply to the redirected one?

I'll try that when i get home.

Thanks