Hi @Michael Herman,
According to your description, I suggest you check whether the user's email address/SMTP and login address/UPN match. Based on my experience, if the two do not match, it will cause login problems:
- Check email address/SMTP: · In the EAC, navigate to Mailbox under Recipient Configuration. · Find and select the corresponding user mailbox, select "edit". · In the "Email Address" tab, you can see a list of all the user's email addresses, including the primary SMTP address, which is usually shown in bold and preceded by the "SMTP:" prefix.
- Check the user login address/UPN: · In Active Directory Users and Computers (ADUC), find the corresponding user account, right-click and select "Properties". · In the "Account" tab, you can see the user's login name and domain, which is the user's UPN. A UPN is usually in a format like an email address, such as "[user@domain.com]".
If you have any questions, please feel free to contact me.