You're probably reading this chapter because you want to write a mail client, or to extend an existing application to work with email. This is a common desire. In fact, as Jamie Zawinski put it in his Law of Software Envelopment:
Every program attempts to expand until it can read mail. Those programs which cannot so expand are replaced by ones which can.
That statement was probably made somewhat tongue-in-cheek, but it does make a point about the importance of email. Email is ubiquitous. Reading and writing email is a vital part of many people's communication, both at work and in their personal lives. In addition, users and administrators have come to expect emailing as an interface to communicate with their applications. When an application needs to remind users of lost passwords, provide a simple way to update a bug-tracking system, or allow for uploading pictures from mobile phones, email integration becomes a requirement.
Twisted provides support for working with email through all of the standard protocols commonly used today: SMTP, POP3, and IMAP.
Building Simple Clients and Servers
Web Services and RPC
NNTP Clients and Servers
Services, Processes, and Logging