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I would advise you to revert the change and check again. Better to open case with Microsoft.
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Systems Engineer

KLOGIT Engineers

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In the end I was missing a TLS 1.0 Reg entry. I re-followed this link and created new reg files.<br><br><a href="https://techcommunity.microsoft.com/t5/exchange-team-blog/exchange-server-tls-guidance-part-2-enabling-tls-1-2-and/ba-p/607761" rel="ugc">https://techcommunity.microsoft.com/t5/exchange-team-blog/exchange-server-tls-guidance-part-2-enabling-tls-1-2-and/ba-p/607761</a>
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Enabled TLS 1.2 over 2 months ago and have setup many users since then, but now we cannot enable UM for a user. I only mention TLS because that is what forums say is the cause.<br><br>Here is the error<br><br>Unified Messaging can't validate or generate a UM PIN for mailbox 'user@domainname.com' : An error occurred while accessing the user's mailbox. Details: The underlying connection was closed: An unexpected error occurred on a send.
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Enabled TLS 1.2 over 2 months ago and have setup many users since then, but now we cannot enable UM for a user. I only mention TLS because that is what forums say is the cause.
Here is the error
Unified Messaging can't validate or generate a UM PIN for mailbox 'user@domainname.com' : An error occurred while accessing the user's mailbox. Details: The underlying connection was closed: An unexpected error occurred on a send.

Enable UM for User

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.