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Removing the account from Mac Mail completely, and then RE adding the account back in should fix the issue.
Here are the instructions to REMOVE account. (next set is to ADD account back in)
From the Mail drop-down menu, choose Preferences.
From the Mail menu, choose Preferences, then click Accounts.
Select the account you want to remove, then click the Remove (-) icon.Result: The following alert displays: The account's setup information, mailboxes, messages, notes and to do's will be deleted permanently from your computer. If you still want to continue, go to Step 5.
if they are not saved on the server.....then yes.
So, be sure to copy your mailboxes, messages, notes or other items to an In My Mac mailbox before deleting an account.
Export your mail box
go to the Mailbox Menu <then> Export Mailbox
Save that file with a name you know...a descriptive name....otherwise you will end up with Mailbox1, Mailbox2, Mailbox3 etc etc
Then after you have removed the account.
Go throght the process of adding the account back in again.
Once you have added the account back in......then you can IMPORT the mailbox you exported earlier.
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