wellI have had this rejected a number of times by the receiver as I have no reverse lookup for my smtp server as it is on a Dyn DNS setup.
1. you may have many client machines (even two is enough to be a pain), so you don't want pop mail, because u end up with half your email on machine a and half on machine b. imap solves this. isp's in general do not do imap. ergo do it yourself.
2. if you have dialup you can regularly dialup and send/receive mail, under programmatic control on the linux server.
3. you have a number of dialup accounts with different isp's and don't want to keep changing the smtp server setting in your email client, you just set it to the linux box and let it bypass the isp's smtp server.
/chris
