Why you (probably) don't want to run your own email server
I just finished four years of running a corporate email server, and here are my thoughts on why I'd never do it again within the context of a young SME.
Before I start: I'm not saying don't learn how to set up an email server, because there is definitely a lot of value in learning how to do it. I'm also not saying don't run one at all, because again, there is value in doing it. This post is more along the lines of why a sysadmin should find the budget to outsource this particular part of the technology stack.
The TL;DR version of this would be: You're not Google and you have better things to do with your time than try to recreate what Google Apps already does for email. Besides, you can't guarantee 100% uptime on your VPS, dedicated server or bare metal email server setup. 100% uptime is a feature, and no business, no matter how small, should choose the potential troubles of running email in house over guaranteed uptime (I ran mine first on bare metal, and then on a dedicated server - both were the same for all intents and purposes, except the dedicated server was quieter).
Email is, of course, mission critical to any business. So are web and database servers. But as a sysadmin, you know what to do when those go down. You have those servers backed up and your code is in repositories and you have a disaster recovery plan that can have you back up and running in no time.
But email doesn't work that way. What happens when someone in an office halfway around the world does something that blacklists your domain? Email servers come with a host of potential problems that planning and backups won't help you with.
That said, running a corporate email server was a great experience for me. I learned a lot. Once I had my setup running, I barely had any problems. Blacklisting and fine tuning spam protection were basically the big ones.
Still, it's something that deserves a full-time solution for any business, and for most sysadmin/devops people in SMEs, your time and expertise are probably better spent on the web and/or software stacks.