When you use bulk import or directory synchronization to import users, users must have email addresses in the source system. Those must be the addresses they use for Cisco Webex because you map them to the Cisco Webex user ID.
If a user does not have an email address in IM and Presence Service, the Message Connector cannot discover the user. Hybrid Message Service does not work for that user.
Using Cisco Directory Connector you can map a chosen attribute (eg. the mail attribute or the userPrincipalName attribute) to the Cisco Webex UID, but the value of the attribute must be the user's email address.