Because a lot of us had to do this. Before the Cloud. And the joke with exchange was. A 40K software package. But you need to spend 80K a year just to have someone run it. So anyone but corporate environments with big budgets could really handle it.
I mean and that was only EMAIL. In windows environments there were a lot of older email server packages out there. I ran one for the longest time. Until it got to the point it got so painful to run because some admin out there decides that he'll be by the book and follow any RBL list out there which happens to host your client's email and they block you. And nothing you can do about it.
So run one if you want to learn to really run one. But the day and age of doing so is pretty much over. If you have under 5000 users. It's cheaper to go cloud. Using my 80K for an Exchange admin cost. More likely you need 2. In case one gets sick or gives you the FU because 80K isn't enough to run your damn server. And again we're only talking about Exchange here.
You want to be pooled up with mail provider that 1) Has decent internal spam protection. 2) Large enough that Joe moron Admin won't block them thinking they're hot shit.
I use Office365 as a license. It comes with Exchange. But I do not use it. I still pay for Exchange at Rackspace as a separate email service because they have Hybrid e-mail accounts. Where i can use non-exchange e-mail. I have a bunch of Non Alias emails i want to use and it would be way more expensive to just buy a o365 license just for email.