I'm sure this seems like a dumb question, but it beats me. I know that on the average *NIX machine, there are 3 ways to send mail.

1. Locally only. Send mail from one user of the machine to another of the same machine.
2. Run an SMTP server on the machine.
3. Do what most graphical email clients do, route mail to some other SMTP server over the web.

My question is, which of these does sendmail (or the postfix replacement) do? Can postfix be used as a drop-in replacement for the "Send Mail" button of a graphical client? And similarly, can fetchmail (or getmail) retrieve the mail from remote servers like the "Get Mail" button, or do they only handle things locally?

Sendmail and Postfix are MTA's (mail transport agents) and they are basically smtp severs that collect mail on the smtp port and deliver it somewhere.
By drop in replacement, not quite sure what you mean, but Postfix can take email as an smtp server and deliver it anywhere (so I think that means yes).
Fetchmail typically fetches mail from remote POP servers and delivers mail locally (or to an SMTP server like Postfix)

Hope that helps

I guess what I'm asking is, when I send mail from a graphical client it forwards the mail from my computer to my universities smtp server, so the mail seems to come from my email address @edisto.cofc.edu . If I were to switch to postfix, would that send the mail directly from my computer, thus making it seem to come from my local IP address? I'd much rather present the prettier .edu address.