Although
Sendmail, Inc. still claims that Sendmail "powers the majority of
mail servers on the
Internet", others have raised doubts as to whether this remains the case.
Dan Bernstein, the author of
Qmail, has conducted
surveys of
mail servers which suggest that between November
1996 and October
2001, the popularity of the Sendmail
MTA steadily declined from 80% of all mail servers to 42%. Bernstein's own Qmail stands at 17%, trailing Microsoft's
Exchange at 18%.
Why has Sendmail declined thus? After all, it is (as always) free software, and is distributed with the majority of Unix and Linux operating systems. Since a fair majority (at least 65%, according to Bernstein) of mail servers run on Unix systems, this means that a significant number of mail server administrators are choosing to replace a pre-installed Sendmail with another MTA.
I suspect that the chief reasons Sendmail has been rejected include security, manageability, and speed; all three ultimately stem from Sendmail's complexity and history. First, Sendmail has an unfortunate history of security holes. Although current versions are certainly much more secure than old ones, sysadmins have long memories and try to avoid getting burned too many times by the same product. Sendmail also has a reputation for being evil to configure, though this is ameliorated greatly by M4. Finally, and perhaps most importantly for large sites, Sendmail is quite heavy compared to many other MTAs. Lighter ones such as Postfix or Qmail can handle higher volumes of mail using less computing power -- and thus costing less money.
Dan Bernstein's surveys are available at
http://cr.yp.to/surveys.html. As Bernstein is the author of a rival (albeit free) MTA, you may wish to take them with a grain of salt. However, they do seem to jibe well with my experience -- and, I'll admit, with my own biases: if you're setting up a mail server, I'd recommend you look into
Postfix and
Qmail in particular before considering Sendmail too strongly.