User preferences don't include all domain users

Hi

FortiMail working in gateway mode usually a user preference is added automatically for each domain user but recently I noticed that not all users have user preference added to FortiMail. I found below note in FML guide but it doesn't mention what events automatically create user preference for each user

Any Ideas ?

FortiMail Guide:

Although you may have created a local user account, the user’s preferences may not be created. You can either wait for an event that requires it to be automatically initialized using the default values, or you can manually create and modify it.

Thanks

Mahmoud