And it makes you a second (if we're being generous)-class citizen to all the mail servers to which you will likely attempt to deliver mail. Good luck getting GMail to accept mail to the inbox of your friends from your TurnKeyLinux MTA in a box or whatever, or just because you managed to apt-get install postfix and get it to communicate with your client. Having run mail servers for a hosting company with tens of thousands of servers, the easy answer for most people is "don't do it, not ever ever".

If you're the type who understands why not to do it, but does it anyway "for fun", that's probably OK because you understand the risks and what types of bad things can happen (and probably have some idea of how to prevent and/or fix them).

If you don't and "just install OwnCloud" or whatever the flavor of the week is for "own your whole social stack by installing linux on that dusty tower sitting in your garage and STICK IT TO THE MAN!", you're likely setting yourself up to get your identity stolen at the very least, with having the FBI come and seize all of your electronic equipment not out of the picture, e.g. in the case of some hacking crew using your poorly-secured box as a proxy / jump host / C&C / worse. No thank you very much.

I know most of this stuff because I've been there, and the people like me who have done it for years and see the horrors are honestly trying to help those who might be tempted. I set up exim with SpamAssassin and clamav on Debian before I could drive with my own bind server hosting all the zones out of the basement, Server 2k3 running AD, Zimbra and slapd + freeradius on opensuse. I did it to learn, and after getting it all set up (which took weeks back then) and getting inbox deliver to gmail (again, in a time when doing so had a much lower bar), I thought I was some kind of Internet wizard. I did learn a ton before my first tech job doing this, and I also learned why it was a bad idea. I got a letter from my ISP a few weeks after getting this all online (of course I had remote access set up via pfsense with SSH on a non-standard port with root login disabled) informing me that there was a botnet C&C running at our home IP. Figuring this was just a mistake, as I was active on IRC at the time, I dug into the traffic and basically found that what the ISP said was true. I was able to set up a mitm on pfsense to get a log of the IRC activity in order to figure out exactly what was going on. Most of my set-up had been taken over without me even realizing it.

Quite a few years later, I worked in web hosting for a few years. The amount of terrible things which happened to unmanaged colos and dedicated servers was astounding. There were quite a few legal disputes which involved data on systems we physically controlled. There were companies which went out of business due to data loss. There were others which were blackmailed by hackers. Others simply went under. There wascredit card theft, databases dumped, unsalted passwords, you name it, and plenty more we probably didn't know about. The Internet was a very hostile place for a server years ago, keeping hackers busy looting and pillaging. I'm sure it's much worse now, with more profit to be made, tons more stuff to be pwnt, and tons more people launching poorly-configured stuff.

Leave the MTAs to the pros. I promise that somewhere out there, someone is getting paid a good amount to spend quite a bit of time keeping that mail server online and in good shape. They are very likely better at this than you, have way more clout with their domains than you (no one cares if your domain / NS / netblock / AS gets blacklisted, but if gmail / yahoo / aol / Comcast stops working, postmasters@ all over will jump to fix it), and are getting paid to do it, likely as a full-time job, likely for many systems. You may be able to set it up once, and make it work a few times, until it annoys someone who cuts it off, at which point you have very little recourse and very little chance of ever making it work as well again.

The point isn't that you can't do it, the point is that it's not worth the risk when there are so many other great options out there.