Hello @Ivo Lindner ,
Thanks for reaching out and apologize for the delayed response.
From your question understand that you have configured Azure AD DS for your environment and enable to access and wondering how to verify if specific users has synchronized to the managed domain.
Ideally, when Azure AD managed domain services enabled, by default, all users and groups from an Azure AD directory are synchronized to a managed domain. So, for cloud-only user accounts, users must change their passwords, wait for 15 minutes before they can use Azure AD DS and here is detailed explanation on why users need to change their password once AADDS was setup.
so, if you have created managed domain say mycompany.com then user object would be synchronized to the AADDS with the UPN format, such as email@example.com and SAMAccountName format like mycompany\user.
Therefore, would request you to verify if any of below cause the issue for your environment while accessing managed domain with secure LDAP also here is detailed guidance on how to troubleshoot secure LDAP connectivity issues to an Azure Active Directory Domain Services managed domain.
Scoped synchronization: If you are using scoped synchronization then make sure user is part of group is part of AADDS sync scope. Reference: Scoped synchronization overview.
NTLM password hash synchronization: If NTLM password hash synchronization disabled on your managed domain, then users (and service accounts) can't perform LDAP simple binds. For more information on disabling NTLM password hash synchronization, see Secure your managed domain.
Cloud-only environments with no on-premises synchronization
Additionally, in order to verify if user account has synchronized successfully, you can try to join windows server VM with managed domain from azure, which is part of same VNET where AADDS hosted, and then you can open Active Directory Users and Computers (dsa. msc) console to verify if given user sync from AAD to AADDS (usually it would be under AADDC OU shown below) also you can use
pwdLastSet attribute user properties to verify last password set time stamp for the user.
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