After decades of running my own email servers I gave up because of the never ending blacklisting. Some random ISP will decide that your IP address is in a bad neighborhood and blacklist you. Eventually one of your users will realize that their mail to Aunt Vickie isn't getting through and you'll spend an hour trying to get unblacklisted. You may or may not succeed.

If you own your own block C or larger this may not apply, but if you are renting servers or using a colo there is woe to be had.