SM version 16.3.6816

Customer site uses MacBookPro on desks and iPhones in pockets.  Macs are using EWS with the Apple mail client, and iPhones are connecting via EAS in the typical Exchange-like way.

We are receiving numerous complaints from client-- their users are reporting that sometimes new messages will appear on their iPhones and Webmail, but fail to appear on the Mac.  It's sporadic-- for example, 100 messages arrive on iPhone/Webmail but perhaps only 98 of them will show up on the Mac.  

This only seems to affect only Mac users.  Windows Outlook/iPhone clients at the same customer site are fine.

None of us are familiar with MacOS so I'm not sure where to start diagnosing.

Ideas?