The solution of isolating exchange mailboxes
When you send an email to a user, you will receive the following error:
Your message was not delivered because the recipient's mailbox is quarantined. If the problem persists, contact support.
Solution:
Log in to the EXCHANGE server and open the exchange management shell.
First use the following command to look at the user's attributes
You'll see IsQuarantinated is True.
Then change it to False with the command
Check again, the client should be fine.
The official explanation is as follows:
https://blogs.technet.microsoft.com/sjimmie/2010/07/09/mailbox-quarantine/
