I upgraded form 10.16 to 10.17 and within a few hours people started complaining that mail was not coming through on their phones.
First it looked like one or two people, purely with iPhones, but after a while it was more.
Now I notice my Android phone is not updating all folders.
Anyway. downgraded back to 10.16 and most issues have gone away and people are getting email again.
Anyone with any ideas ?