Mac Mail not Syncing with Exchange email server on MacOS devices

I have the same issue, and all of the proposed solutions (like powering down iOS devices) either do not correct this or do so temporarily. This is an intermittent problem, which makes it so difficult to diagnosis by this type of trial and error approach. Sometimes a solution seems to work and it is likely just a coincidence. Apple really needs to work on this; there are multiple threads on this issue so it is a major problem with functionality of Mail.

Thanks for your quick reply. Out of the items in your list, the only thing I have not done is delete the Exchange folders. Maybe that will be the secret sauce this time. Definitely will give your checklist a try and will let you know the results in a few days.

While I agree that a large quantity of messages in the in-box seems to be part of the issue, since I upgraded to Big Sur I have consistently spent time every night clearing out my in-box. When it lost connection at 4:55 am this morning, I had 24 unread messages and 7 read messages in my in-box. Coincidentally, at 4:55 am I had the exact same number of messages in the InBox in Outlook for Mac and the iOS Mail App on my phone and those programs don't have any problem connecting with Exchange. At this point in the 21st century, it should be no surprise to the developers of the Apple Mail program on Mac OS that many people are buried in a daily avalanche of emails.

This is a very helpful forum for discussion of this problem. I also wanted to emphasize the bizarre intermittent nature of this issue for anyone talking with Apple. I have had exactly the same problem that has been described here using Catalina. But yesterday it flipped back to no issues at all. It is syncing perfectly, functioning just like my iOS mail. I did nothing to cause this. I imagine that one day it will stop working again, unfortunately. But for now it is a nice reminder of how sweet it is when it actually works as it is supposed to. 

I am having the same problem.

The issue occurred just before the latest update. I updated the system, it worked and now a week later the problem occurs again.

I am also using a new iPhone 12 mini for my emails, I hope this isn't causing the issue (I doubt it).

I too have a large number of emails and am receiving 100 per day so this issue is highly frustrating.

Please can Apple sort this out.

I cannot keep shutting down and reopening my Mac every hour!

Today mine is miraculously working again and I have done nothing except stop checking my emails on my phone?!

I am purely using the mobile for calls whilst working at my desk, so less requirement for both devices to refresh. Perhaps browsing emails on the mobile is a habit to be avoided.

Just try sticking to one device at a time.

I also wondered if it gives up the sync when the demand on the wifi is very high (extra people working from home in the area, school kids streaming etc). This seems to be the only explanation???

It would be nice if an Apple expert/developer could comment on here and clarify this.

and out of nowhere also my iMac suddenly stopped syncing my Exchange inbox again....That's now 20 hrs ago and the only way to receive new mail (other than via iOS devices) is restart the Mac. Or using Outlook which for me is halfway going back to windows again.

great all these fantastic Apple products, watch, new glasses and the like but they better make email work or else its all useless to me. Apple in a corporate environment just s@cks i think.

I havent bothered yet to start reconfiguring the Exchange account again (as this usually works). Many years ago I made the switch to Mac for the ease of use and reliability but it starts to reverse, especially when i read how many users have been experiencing the same issue and with Apple doing zero. I will now just wait and see what happens and meanwhile use Outlook. Maybe i will get used to using Windows again

Yep, mines the same.

Nothing at all yesterday, this morning it’s been behaving perfectly, and I’ve been accessing email on my phone as well so it’s nothing to with that.

not going to ask why, I’m just happy it’s working again and I can stop with outlook!

Thanks to everyone for all the assistance

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

@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.

Moving from junk to the Inbox syncs.

After deleting Junk from Mac, the sync had some issues on the server side. It took a while so, while spinning wheel on server side, I did delete a single email in the Trash on the server and this broke the sync. It deleted the item on the Mac, but the items that had been moved from junk to Trash ij the Mac remained yet the server stopped the sync and showed nothing.

Then, deleting from the Mac inbox would sync but not from server to Mac. And, when I went to delete that email on the Mac, I received the error below.

But, I could move it to another mailbox (not with the server account).

After a bit, large moves of files on the Mac would break the sync on the server.

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

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

Using Big Sur, Mac Mail 14.0