SMTP servers are often referred to as mail transfer agents (MTAs). These systems potentially function as both clients and servers in a mail transfer chain. An MTA can accept mail that's sent by a user or by another MTA. It can then store that mail locally and, if necessary, send it on to another MTA. There are several possible uses for a Linux MTA:
These functions are so important that most Linux distributions install an SMTP server by default. In particular, the local-to-local mail delivery issue makes the presence of an MTA on a Linux system very desirable. (In theory, you could track down everything that tries to send mail locally and reconfigure or rewrite it to use a remote SMTP server, but this may not always be practical.)
Most distributions' default SMTP server configurations are adequate to handle local mail delivery. The question therefore becomes one of when you should alter or at least check that configuration so that the computer can take on additional duties , such as handling mail for an entire domain. E-mail is so important in today's world that few networks can do without it. Even within a small office or a home, e-mail can be an important communications medium. Running your own e-mail server can be a way to provide e-mail, but it's not the only way. Outside e-mail providers are not uncommon, and you may already have access to an outside supplier. Such providers can be a good choice for receiving mail for a small domain, if you don't want to be bothered with creating such a configuration locally. Running your own e-mail server provides you with more control over how it operates, though. For instance, you might be able to more quickly and easily add, delete, or modify users if you run your own server. You can also configure your own server to block unwanted e-mail, adjust quotas on the size of messages the server will accept, and so on. Running your own server may be less expensive than contracting with an outsider to do the job. As a general rule, you may want to run your own mail server to handle the bulk of your mail needs if your network hosts more than a handful of users or if you have particularly exotic e-mail needs.