You mention this is an IMAP mail account, so evcerything is on server. Have you tried removing and reinstalling the account.? - Access Account Settings - select outlook account name - click on 'Account Actions' - select 'REmove account' - Exit Thunderbird - Wait a few moments for backfround processes to complete. - Start thunderbird. - Create New > Existing mail account.
Bug 1852814 Comment 5 Edit History
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You mention this is an IMAP mail account, so everything is on server. Profile was imported - so this account has not been created as new. Have you tried removing and reinstalling the account.? - Access Account Settings - select outlook account name - click on 'Account Actions' - select 'REmove account' - Exit Thunderbird - Wait a few moments for backfround processes to complete. - Start thunderbird. - Create New > Existing mail account.