Communication by email is probably the most pervasive and most compelling aspect of using the Internet. There are two ways that you can get your mail from a UNIX "post office" that differ in a subtle way.
The first method requires you to directly contact the computer that receives and holds your mail and read it directly. The advantage of this method is that your mail always remains on only one computer. No matter where you travel in the world, all your mail will be available to you when you contact your computer. You will usually talk to your computer with a simple terminal-like program such as Telnet and get your mail directly off the Unix host with programs such as "mail" or "elm" or "pine" to name a few.
The second method is to use a program on your local microcomputer to contact the UNIX host, download your mail and then disconnect. All of your mail will now be located on your local machine and not on the remote UNIX host. The advantage of this approach is that you now have all mail locally. These programs usually have quite a bit more capability and ease of use than those described above and most people prefer them because of this. The only disadvantage is that you cannot "wander" from computer to computer and still have all your mail to work with. Your mail only resides on the microcomputer that downloaded it from the UNIX host. Here are a couple good mail programs for your Mac.
Eudora Lite can communicate with a remote "post office" computer and help you organize and send mail. It will allow you to easily respond, forward, and archive your mail. Eudora will also let you attach enclosures (files) that could be word processing documents, spreadsheets, graphics, or anything you can store on your local computer. It will also receive these enclosures and store the file on your disk for you.
Claris Emailer is another excellent (and free) email management program. Emailer does everything that Eudora can do and more. Emailer can manage your mail from several different accounts (AOL, Compserve, Internet...). It can also direct mail to different folders based on the contents of the "From:" line or even the "Subject: " line in the mail message. This means you can presort your mail, putting things like listserv messages in their own folders. If you need more advanced mail management, Emailer may be for you.
Both of these programs are are free. Both also have not-for-free siblings that have even more advanced features.