The installation procedure installed
exim, a Linux mail transfer agent. Therefore, you should be able to send email to users on other systems, and to receive email sent by them to you. Simply reconfigure your favorite mail client to specify your Linux system as your mail server. For example, if you're using Netscape Navigator, select Edit 
Preferences Mail Servers. Make a note of the existing mail server settings, just in case something goes wrong. Then, change the incoming and outgoing mail server options to specify your Linux machine.
If you want to continue receiving mail on another mail server, but want to read it from your Linux system, you might consider using
fetchmail, a package that can retrieve email messages from the server (by using POP, IMAP, or just about any other remote mail protocol) and forward them to your Linux system.
