Somehow e-mail lives on despite spam, instant messaging, Twitter, Facebook, and LinkedIn. Heck, if it weren't for e-mail notifications, I'd never check my Facebook page. But what's driving e-mail these days?
A look at MailRadar.com shows that Sendmail is still the No. 1 MTA (mail transfer agent) in use today, followeded by Postfix, while Qmail is a distant third. And dear God, someone is still using MMDF. Surprisingly, Microsoft Exchange isn't even in the mix, which casts doubt on the validity of those numbers, but it's probably safe to say that Sendmail remains the top MTA.
The rallying cry behind most non-Sendmail MTAs is that they're, well, not Sendmail. Sendmail is much maligned for being a security risk, but my experience of running hundreds of Sendmail-based mail servers and mail relays does not bear that out. Over the past 15 years, I've yet to see Sendmail used as an attack vector.
Case in point: I recently helped a friend set up MailMan on a hosted Linux VPS server. The VPS ran Plesk and Qmail. Seems simple enough, except that in this particular case, e-mail for the domain was hosted elsewhere, so a subdomain was required to push e-mail traffic to that specific server. Still, that's a very minor configuration issue, or so I thought. After several hours of diagnosing perplexing Qmail errors, I switched the whole server over to Sendmail and had everything running in a matter of a few minutes. Indeed, my familiarity with Sendmail assisted here, but I'm not a Qmail newbie either -- there are some significant problems related to Qmail and Plesk in this instance, and I simply wasn't willing to fall all the way down that particular rabbit hole.
Another stick poked at Sendmail is that it suffers from Byzantine configuration. Speaking as someone who's written custom rulesets for a hideously complex Sendmail structure, I can verify that when you start digging into the guts of Sendmail, it gets crazy real fast. But the point is that you really can dig into the guts. Like many things in computing, high configurability comes with high complexity. The reality is that 99 percent of Sendmail configurations are extremely simple -- a few lines in a sendmail.mc and a make and all is well.