Explanation/Reference:
Explanation:
Azure AD Connect lists the UPN suffixes that are defined for the domains and tries to match them with a custom domain in Azure AD. Then it helps you with the appropriate action that needs to be taken. The Azure AD sign-in page lists the UPN suffixes that are defined for on-premises Active Directory and displays the corresponding status against each suffix. The status values can be one of the following:
State: Verified

Azure AD Connect found a matching verified domain in Azure AD. All users for this domain can sign in by using their on-premises credentials.
State: Not verified

Azure AD Connect found a matching custom domain in Azure AD, but it isn't verified. The UPN suffix of the users of this domain will be changed to the default .onmicrosoft.com suffix after synchronization if the domain isn't verified.
Action Required: Verify the custom domain in Azure AD.
References:
https://docs.microsoft.com/en-us/azure/active-directory/hybrid/plan-connect-user-signin