In some cases, you may see the following error message in Easy365Manager:
This error message indicates that the domain suffix used in your on-premises userPrincipalName does not match a verified domain name in Office 365.
Select the Account tab to view and modify your on-premises userPrincipalName:
Configure a domain suffix that matches a verified Office 365 domain name. Then synchronize the change to Office 365.
If this is a general problem in your environment consider using PowerShell to automate proper alignment of your userPrincipalNames.
To read more about the importance of proper alignment of the userPrincipalName attribute, read this article.