In mail on a iMac for one of my email addresses I get the following message when I try to delete a message ‘An error occurred while moving messages to mailbox "(null)". Black square symbol appears in front of the sent and trash email.

In mail on iMac for one of my email addresses I get the following message when I try to delete; ‘An error occurred while moving to mailbox “(null)”. There is a square symbol in front of the email address in...
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