Outlook 365 gives 'target principal name' error. Server is a shared host I administer. Cert is for only server's own domain, not hosted domains, changing mail server name to match that causes it to refuse to allow IMAP or SMTP login to server.

Geoff Roberts 0 Reputation points

Changing the mail server name to match the server name doesn't work - it won't login to either imap or smtp if you do. I can't run unencrypted ports because hosts like gmail will not allow mail from servers where user is not authenticated to the server. This creates problems for my domain customers who need to send to gmail amongst others. It would be ridiculously expensive to get certificates for all the domains on the server and I'm not even sure if that's possible since they all use the same mail server.

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Outlook Management
Outlook Management
Outlook: A family of Microsoft email and calendar products.Management: The act or process of organizing, handling, directing or controlling something.
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  1. ChristyZhang-MSFT 21,181 Reputation points Microsoft Vendor

    Hi @Anonymous ,

    Welcome to our forum!

    Do you receive the below error?

    If so, based on my research, i found a similar thread as yours, it's suggested that you could try the solution as Craig mentioned in it and check if the issue can be resolved:
    Outlook 2016 says "The target principal name is incorrect" on my site's security certificate
    (Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)

    Hope to help you!

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