Are you still using your own server for sending and receiving emails?
We would recommend using either Microsoft or Google for sending and receiving mail – that’s the best way to avoid getting flagged for spamming, not receiving some mail and not having the server as the single point of failure. It will be the same on your end ie Outlook, calendar etc. We would be keeping all the email addresses and mailboxes – everything will look the same except we would be using Googles infrastructure to manage emails (faster, more reliable, Google less likely to miss or get considered for spam). The only downside is it will take a while to ‘propagate’ the switch, meaning that you might have a day of no emails so I’d advise doing it on a weekend.
Taking the email management responsibility off your server should also significantly improve it’s performance in the other areas like file access and updates deployment etc. Should also make it last longer too as it will not be using the hard disks as hard or as much.