Reasons that come to mind:
- Fear of improperly configuring your mail server, contributing to spam/relaying spam.
- Fear of ending up in people's spam boxes?
postfix + opendkim + smtp + tls + spamassassin + fail2ban is not too difficult a setup to achieve...
Sending them such that they don't get spam-canned is way more difficult than you're admitting. It can take months or years to get decent accept rates, all of which can be lost by a simple misconfiguration.
Parsing received emails if you want to do anything programmatic with them is also very difficult. The specs for email are vague, and even then many providers break them regularly. Want to strip quoted replies or signatures? Now you have a machine learning team, too.
Simply put, sending and receiving email is way cheaper to pay someone dedicated to the task to do. You have to get to a very large scale before it's worth doing yourself.
When it comes to side-projects, mail servers fall into that area where they're distant enough from the thing I'm building to warrant just outsourcing that activity, especially with the number of transactional email providers offering free tiers.
They provide a fairly easy to use api, and a graphical user interface to monitor sends, they automatically stop sending if a user requests so you don't get marked as span. Of course that happened to us twice, one time the user then inquired why they weren't getting the email alerts....
For us our public facing information website, was controlled by the business dept, not the same as our functional site, done by the engineering side on AWS (before AWS had mail...). So it allowed us to send from the main domain without being put right in the spam folder.
I used Mandrill for a small non-profit site, very low volume, so I'll be moving to probably one of these providers.
Yes, here's the reason people don't run their own servers!
> is not too difficult a setup to achieve...
Yes it is, for a lot of people. Not to mention security patches for all this software.
Add that to the fact that Google reportedly is reportedly aggressively classifying mail from new servers as spam (your point 2) and you greatly increase the transaction cost required for people to run their own mailservers.
If I was running a mail service company of some sort then of course it's easy enough to do this. Unfortunately time, effort, focus and money is best spent elsewhere in the company. Maybe one day, but for now it's far easier to scratch out 3 of the 4 resources by throwing money at the companies that do this sort of work.
Especially when you start looking at features like email->HTTP POST request with parsing built in.
I've run my own mail server for quite a while and might do it again for inbound, but sending just is annoying to monitor and keep up.
Setting up a mail server is often worth it, but requires experience that about 0.01% (being generous) of the developer community has.