High Sierra Mac mail not syncing with Exchange/Office 365

[yoricardo]

About a week ago my 2011 iMac running High Sierra (cannot update to newer than that) stopped syncing with my Exchange/Office 365 account. Strangely it seems to sync my junk folder, but not my inbox or sent items.

I can send from the account and that sent email then appears on my other devices.

If I rebuild the mailbox all emails comes through ok however.

If I use Outlook online my mail is fine, as is my iPhone and iPad. My 2017 MBP running Big Sur is also fine.

Coincidentally I spoke to a friend today who has the exact same problem, running the same OS.

Spoke to Apple who say that as my iCloud emails are fine, it must be a Microsoft problem. Spoke to Microsoft who said that because Outlook and my other devices are fine, it must be an Apple problem.

Does anyone have a solution?

 

[317342]

I have three exchange 365 accounts in Apple Mail, two of which are enterprise. I can tell you that all three of them behave very differently.

I got an alert on one of them (work/edu) about a week ago that a certificate was expiring and they wouldn’t be compatible with older third party clients, naming Apple Mail.

I’ve had no other issues but all three Exchange accounts are usually glacial when it comes to syncing.