IMAP is a newer email service, supported in Netscape clients beginning with Communicator 4. With POP, you either leave your mail on the server (you cannot delete or rearrange it at all), or you remove it to your local machine...neither of which is really what you want when you have multiple access points to your email. IMAP, on the other hand, allows you to keep the mail on the server machine but at the same time manipulate it (drag around in folders, delete, etc.) This is great for situations where you access email from more than one machine but want it to reside only on one.
Example: my email will remain on my IMAP server machine (my workstation at work), but I can use it, see all of the directories, etc., from itself as well as my home workstation, my laptop, and my desktop PC.
Server A (SGI system) will use IMAP. The new and unsaved messages will stay in /var/mail and messages were stored in ~userid/nsmail directory. Server A will be the IMAP server and a client as well. To do this, as super-user or root: