Symptoms
Consider the following scenario:
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You are running Microsoft Exchange Server 2019 CU13.
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You have Modern Authentication (Hybrid Modern Auth or ADFS Modern Auth) configured.
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The primary SMTP address differs from the User Principal Name (UPN).
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You use an authentication policy to enable Modern Authentication on a per-user basis.
In this scenario, Modern Authentication is enforced for users, even if BlockModernAuth is set to $true.
Additionally, if you add public folders to your Favorites folder by using Outlook on the Web (OWA), the operation fails and returns an "HTTP 500" error message.
Resolution
To fix this issue, install the following cumulative update:
Cumulative Update 14 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019
More information
For more information about how to enable Modern Authentication on a per-user basis, see the "Install Exchange 2019 CU13 on all FE Servers (at least)" section of Enabling Modern Auth in Exchange on-premises.
