Quite a few SMTP servers are available for Linux. The four most popular are:
Although these are the most popular Linux mail servers, there are others, such as Smail (http://www.gnu.org/software/smail/smail.html), Courier (http://www.courier-mta.org), and OpenMail (http://www.openmail.com/cyc/om/00/). Most of these are open source, but some are commercial. The "big four" account for most of the discussion of mail servers in the Linux community. All four of these programs are very powerful and able to handle the mail needs for most domains ”even very large ones.
Particularly if you're new to mail server administration, you're probably best off using whatever SMTP server shipped with your distribution. Many distributions now include multiple SMTP server packages. The default is probably the best choice.
If you have specific needs, and particularly if you have exotic needs, you may want to investigate the capacity of various mail servers to meet those needs. If necessary, you can replace your standard mail server with another one. Because of sendmail's popularity, this usually means replacing sendmail with another server. Exim and Postfix usually work well as "drop-in" sendmail replacements . Although the configuration files are completely different, programs that call sendmail directly usually work well with Exim and Postfix, and the mail queue format for these two programs defaults to the same format that sendmail uses ”namely, the mbox format, in which all mail in a single mail "folder" is stored in a single file. Replacing sendmail with qmail is usually a bit more involved, because qmail defaults to a different mail file format (the maildir format, which uses a directory in which messages are stored as individual files), so you may need to change the standard qmail configuration or replace your mail programs (including any pull mail servers you want to run, as discussed in Chapter 11).