Mac Mail not Syncing with Exchange email server on MacOS devices

interestingly I just noticed that not only the mail app on my iMac but also the mail app on my MBP stopped syncing my Exchange account. Still good on Outlook and iOS.

So it seems not be something locally. I am thinking it could have to do with icloud keychain, although that would not explain I can still send mail and connection assistant indicates I am still connected with the exchange account

I think I finally fixed this on my Big Sur M1 Mac Mini by deleting the MailData folders, but probably more importantly, using OWA and Outlook to move messages out of my Sent folder and one of my main folders that I use for filing messages. Both were over 10,000 messages so I started filing messages in other folders by year so my main Sent folder and archive folders are smaller. There shouldn't be a hard limit on the number of messages in a folder, but this has helped me in the past and appears to have fixed this once again. It's incredibly annoying.

UPDATE: I turned off my phone and restarted my computer and all is magically working perfectly again. Go figure. My guess is some of these replies where it starts working again spontaneously, the user turned off their phone sometime in the course of the problem but didn't make the connection to the resolution of the issue on the computer. Anyway, I'm sure there are many sync problems this doesn't solve - but it solved mine! Thank god for this forum, otherwise I would have been trying to delete libraries and rebuild caches or all kinds of other things I probably would have screwed up. :)

@SingingFriar - has this fixed continued to work? I also have a couple of folders with close to 10,000 messages in them (sent items, and one where automated system reports go). I cleaned these up, deleting messages or moving to subfolders, and Mac Mail still won't update. I've tried deleting / recreating the account in system prefs, and I've rebooted, but still nothing.

So far, so good. I did delete the entire MailData items for Exchange and needed to move the messages in OWA or Outlook. Slowly but surely my Mac caught up and has been more or less normal for the past few days.

Still no good for me. Having got all folders down to sensible numbers of messages, I removed all of my accounts from internet accounts, deleted the MailData folder from ~/Library/Mail/V7 (and the mailbox holding local items, just for good measure). And, just to be sure, rebooted, cleared the SMC and the PRAM. Added my work Exchange account back... and it synced all the way back up to where it had previously stopped and now won't acknowledge any new email. As before, "Rebuild mailbox" works sometimes as a one-off, but still won't sync.

Maybe this is a Catalina vs Big Sur issue now, that the fix worked for you and not me. Or just something else.

Thank you for all of your posts! I was struggling to get my Exchange mail account to work on my new MacBook Air running Big Sur 11.2. Nothing worked (deleting account and recreating; turning off all Apple devices; a call to tech support; sending myself an email from Mail). Finally, I opened outlook in Safari and deleted the thousands of files I had in bulk mail, junk, sent, drafts folders. I also deleted some mail folders to clean everything up. I synced my mail, rebuilt the mailbox, and restarted Mail several times. Finally, it worked! I believe the issue was with having too many emails in Exchange to sync.

Do you have the apple mail already up and running again? I have the sam problem and it started out of nothing a few day ago.

same here, miraculously it works again.....So far, so good.

How many days did that take?

I guess 3 days or so

I've been experiencing this problem as well and tried rebooting but still had issues. I then tried sending an email FROM the account that wasn't syncing and then all of my new mail came in.

This is unbelievable but true - switch your phone off, and bing, mail arrives on your laptop.

Good spot @tresinnoctem - thanks.

Tried turning off my iPhone but did not work for me.

Using Big Sur, Mac Mail 14.0