Imagine your website suddenly goes down. Not a single page will load. Maybe your traffic is small and the downtime isn't a huge deal. Or, maybe your site brings in hundreds of dollars a day. Ouch.
Pretty bad, right?
Now imagine that, along with your site, your email for that domain/website is also down. They're both on the same server and whatever has brought your server down means you can't send emails and your customers can't get emails out to you.
Now that's a real website emergency. And it is painful. That sort of scenario - which isn't all that uncommon - is a perfect example of why your email and your website should be hosted on separate servers.
But let's get into it a little more, shall we?
If one goes, they all go
As the previous scenario demonstrated, if your site and email are on the same server and your server goes down, it all goes down. That should be enough of a reason right there to separate the two.
If your email is hosted on a different server and your site goes down, you can at least get business emails out and in. This goes quite a way for controlling what could otherwise be a disaster on your brand perception and bottom line.
Switching hosts is a pain
It happens. Sometimes you just need to switch your web host. With an experienced developer this is infinitely easier on you. But it becomes more complicated if your email is tied to that hosting server.
While it's not impossible, migrating email accounts from one server to another is a pain. It's all too easy to get something wrong and - poof! - a bunch of emails to or from you disappear into thin air.
If your email is hosted on a different server though, you or your developer never have to touch your email accounts. That's just one less thing for either of you to worry about.
Web hosts aren't necessarily good email server admins
The web is a complex place with an array of specialties. Specialization is generally a good thing. You want your web host to be focused on keeping your server running and updated. You don't want them to just go through the motions of the email hosting aspect, which is it's own specialty with its own array of complexities. Is there some overlap? Yes, absolutely. But it's a rare unicorn that can do both really well.
Splitting up your email and your website onto different servers means each host can focus on what they do best.
Bad neighbors in a shared hosting environment
If you're on a shared host, your site's reputation could suffer if someone else on the same server sends spam emails out. Since you're sharing the same IP address, if they get blacklisted for doing not so good things, you get affected too.
This is also a good argument for using a VPS instead of shared hosting, but it's usually out of budget range for smaller businesses. But keeping your email on its own server is definitely doable.
Where to host your email
So if it's a good idea to host your email on a separate server, where do you go?
There are a few of options:
Fastmail. This is who I recommend. And while that is definitely an affiliate link, the fact is, since I switched to Fastmail, I haven't had to think much, if at all, about spam or missed emails. It's just a solid service that is privacy focused.
Google Business Apps. Gmail is a great email provider. Paying a few bucks a month to get your domain onto their email platform is not a bad idea. Their reliability and anti-spam measures are top-notch.
The caveat? Google doesn't get high marks for privacy. So if you don't care if your emails might be getting read (particularly for advertising reasons), Google is a good enough choice.
Rackspace. Rackspace is a relatively reputable company. I used to use Rackspace for my email and while I still recommend them, it's with a major caveat: I don't find their spam filters to be very good anymore. In fact, I helped a client switch away from Rackspace to Fastmail because she also started to get far more spam than before.