macOS mail synchronisation to office365 servers – MacRumors

Anyone seeing periodic loss of synchronisation between macOS mail and office365 mail accounts? New email still arrives, but the server looses track of any emails that have been deleted or moved in the macOS mail client. You don’t notice for a while unless you are also reading the mail online at or on an iOS device as well. Diagnosis is littlesnitch showing that instead of sporadic network traffic to, there is a continuous steady traffic at around 25kB/s upload and 50kB/s download – can be confirmed by setting the offending account to ‘offline’ at which point traffic drops to 0; and by comparing the number of messages on the server in the Inbox (Settings->General->Storage).
Just occasionally select ‘Synchronise’ on the macOS mail client rescues the situation, but most of the time its necessary to delete the account (System Preferences->Internet Accounts) and then recreate it. Depending on the amount of email you have on the exchange server it then spends ages restoring. Of course calendars need to be resynchronised as well.
I first noticed this problem on 12.x. I don’t think its a problem with a specific outlook server as I have 3 accounts with 3 different organisations and they have each lost synchronisation at different times – happens most frequently with the one I use most.
Have reported to Apple ( but not response so far.
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