I have my GMail linked to non-gmail email accounts so that I can send mail as them. Is there a way to use this feature without the web interface (smtp)? I use Mail for Mac OSX Snow Leopard.
[Edited, see comments]
In the Mail application you'll want to add a second (or third, etc. if you have more) SMTP account. Then, when composing an email, you can select which account from which you wish to send it. More information: