Email is a particularly important part of most networks' functioning. Many businesses rely on email for both internal and external communications. Naturally, then, Linux can function as an email servera computer that receives, stores, and forwards email for end users. As part of this function, a Linux mail server can filter out spam and worms from email, thus reducing both nuisance factors and security threats. Linux's advantages as a mail server over Windows include the low costs of the server and of add-on filters for spam and worms, as well as Linux's reliability and immunity to the Windows-based worms that are such a problem today. Even if you already run a Microsoft Exchange email server, Linux can be an excellent supplement to this server, providing filtering features that might require paying extra to obtain in Windows.
This chapter begins with a look at common email protocols and some common server software to implement them on Linux. Most of the chapter is devoted to basic configuration of the sendmail and Postfix servers, as well as to additional servers that can be used to deliver mail to clients. Filtering mail for spam, worms, and viruses requires its own coverage, as does a tool that can help users integrate mail delivered to outside ISPs into their own local mail systems.