Pull mail servers are frequently installed on an organization's main SMTP server in order to allow users to read their mail from client computers located within the organization, or potentially anywhere else on the Internet. Using a pull mail protocol obviates the need for users to log in using Telnet, SSH, or some other remote-access protocol, and allows users to use mail readers in their OS of choice. Two pull mail protocols are in common use today: POP-3 and IMAP-4. Of the two, IMAP is more powerful, as it allows retention and organization of mail on the server; but this very fact may increase demands for disk space and network bandwidth on the server, so you may prefer to use POP.
The Fetchmail program can be used as a bridge between a mail server to which you have access through a pull mail protocol and other mail systems, such as a mail server on a LAN. You can use Fetchmail to retrieve external mail and inject it into a local mail queue, simplifying users' mail experiences.