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If an account is inactive
A disabled email account is dimmed in the list of accounts in Mail preferences, and its mailboxes and messages aren’t shown in the Message viewer.
Do one of the following:
Turn on accounts in Mail: Choose Mail > Preferences, click Accounts, select the inactive account, click Account Information, then select “Enable this account.”
Turn on accounts in System Preferences: Choose Apple menu > System Preferences, click Internet Accounts, select an account, then select the Mail checkbox.