Email is super-important these days to just about every business, so it's a good idea to remain as flexible as possible in this regard.
One solution is splitting your email domain users between SmarterMail and Office 365.
You can complain about the expense of O365, but you do get the entire Office suite including desktop and cloud apps for $12.50/month (retail), with 50GB of email storage and 1TB of cloud storage, and all updates are included. The Office suite is Microsoft's not-so-secret weapon; Microsoft lets you install it on up to 5 devices per user. And there's a range of options starting at $5/mo. that can be applied per user, including email-only, apps-only, cloud-only etc. BTW there's a 300-user limit for some of these plans, and then you're going to the Enterprise plans. But I deal with smaller companies; that's not an issue for me.
And if you have just ONE user on O365 email (at around $5/month) (!), you can use EOP (Exchange Online Protection) with spam & anti-virus (etc.) filtering for your entire domain, for all email incoming and outgoing. And this is all fully legal within the terms of your O365 license, as of this writing. I double-checked it twice with Microsoft before I went ahead with this; it's now implemented for one of my clients, and is working fine. Email flows between all domain users on SM and O365 without issue. I can now migrate users from SM to O365, and vice-versa, when needed.
Takes quite a bit of grunt-work to set up, but that was expected going in.
Calendars are another story. Still working on this but in this case it's not a deal-breaker.