One of the most important compatibility factors to consider with email is the mobility question. How often do your employees need to access email via mobile devices? That's an important issue because most email hosting providers deliver some kind of web client usable as a default inbox. Almost all of these can be accessed via a mobile device, so if your employees don't need to access their emails on the road that much, then such mediocre clients are probably fine.
If privacy is your number one concern, perhaps you should go with Protonmail. You can upgrade to an email with your domain easily on a web interface under “Protonmail Plus” or “Protonmail Visionary” plans. However, the plans can be rather expensive, as getting only 20 GB storage space could cost you at least €24/user/month/yearly. Most email hosting providers retain your emails for months even after you delete your account. This Swiss email provider will destroy all your data when you delete your account.
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