In a comment on my entry on universities, email, and the issues of running things in house , I mentioned that our departmental email system has a non-trivial cost in both hardware and staff time. For the hardware costs I can easily count servers , but things are more fuzzy for the staff time side of things so I'm not going to try to come up with a number (especially because our email system itself is somewhat complex because we have a number of unusual features, such as our simple mailing lists ). The bigger issue is that the basic time cost to maintain your email system is in some sense an illusion, or at least an inadequate measure.

The reality of modern email is that to run a modern email system you need to know about a lot of stuff, and worse the stuff that you need to know about keeps evolving. You can't set up a mail system and then walk away from it apart from software and hardware upgrades; instead, you have to keep on top of a perpetually changing and evolving landscape of anti-spam systems and especially what you need to keep your outgoing email being delivered. SPF, DKIM, DMARC, DMARC alignment, and so on are merely today's names; in a few years there will be more, different things that are necessary to know and deal with, which will require you to modify your mail system as part of its obvious time requirements.

Obtaining this knowledge and keeping up on developments in email is often more or less invisible time. Without it, your system may work today (especially if you set it up from a good guide) but things get riskier as time goes on. At best, you notice when outgoing email starts bouncing; at worse, you have no idea until people start complaining to you (if you're lucky) that email they send isn't getting received properly. Then, if you're lucky, you can find another guide for the current new reality of email; otherwise, you're in for research and learning on your own. To copy from an old story, your time spent might be half an hour to make the eventual change to your mail system but a day of research to know what to change and why.

(A similar thing exists with anti-spam and anti-malware handling for incoming email. In a modest sized environment (such as ours) you have no realistic choice but to outsource much of that to some free or paid piece of software, but that still leaves you to monitor the overall situation, determine when your current software is falling behind, and then find and configure new software.)

Of course you need to develop and maintain a certain amount of expertise with your mail software, but that's usually easier to keep current. Your mail software typically makes changes less frequently than the overall Internet mail environment does, and those changes will often be explicitly documented in release notes, news entries, and changelogs. Still, depending on how much complexity you opt for in your mail system, this may take a significant amount of time initially (learning Exim was in no way an overnight thing for me, and then I had to design and build our configuration because Exim is a construction kit ).

( We have what I certainly hope is an unusually complicated mail system ( also , also ).)

If you have a multi-person system administration team and only one person from it goes through all of this and learns all about email (both things like DKIM and things like your local mail setup), what you've got is a single point of expertise. If that person is on vacation and a mail problem comes up, you kind of have a problem. To avoid this, you'll need more staff time for more people to be at least somewhat up to speed on email stuff and how to deal with problems. At one level this is no different than any other system you operate, but those systems probably evolve slower than Internet mail does and require less time to keep up on.

None of this is insurmountable. But it's not trivial either. Maintaining your own mail system (in the broad sense) is going to take a non-trivial amount of staff time for one or more people to keep up on Internet mail developments, monitor your mail system for signs of problems like too much incoming spam and malware getting through or too little outgoing mail reaching its destinations, and troubleshoot issues. This need for maintained expertise is part of why it can be simpler to outsource email to specialists who deal with it a lot.

(Also, as you deal with a larger population of people using your systems, you also need to worry about compromised accounts sending out bad stuff.)