Create user accounts on Local Accounts page for each domain. User account parameters are set as follows:
User authorization via LDAP
Specify password in account settings to enable authorization at the mail server. There is no need to specify password if the mail server is installed on the computer included in the Active Directory domain and LDAP Sync option is enabled in the mail domain properties. The domain password will be used for authorization.
Figures 1, 2 and 3. User Account Settings