Office 365 - Apple Mail Error: The message could not be saved
If you are getting this message (shown below) popping up regularly on apple mail

Perform the following steps to stop it from happening:
1. Open Apple Mail
2. Go to Apple menu up the top and click "Preferences"
3. Click the "Accounts" tab up the top
4. Select the problem exchange account from the left pane
5. Untick "Store draft messages on the server" as shown in the screenshot below.

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