When your Alias and SamAccountName are different, you can’t log on a POP or IMAP client by using NT LAN Manager (NTLM) authentication in a Microsoft Exchange Server 2016 environment.
This issue occurs because the NTLM method logon is trying to locate you by SamAccountName, where the username is set to your Alias.
To work around this issue, select one of the following methods as what you want:
Don't use NTLM authentication to log on POP or IMAP client.
Set the same value for both Alias and SamAccountName.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
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