Exchange Hybrid + Mail User Object in EXO

[afr2050]

Hi community

I have a AzureAD connect with Exchange Hybrid config (EX19CU10) and around 100 users.
All mailboxes on premise. No plan to mass migrate to EXO at this moment. In preparation.

My problem is, not all on premise users have a 'mail user' object in ExchangeOnline available.
Few users have, few not - don't know why.

1. Why not all on prem users have a 'mail user' in EXO? Should'nt this be the case?
2. How can i create mail users in EXO for the corresponding on premise mailbox?

Thanks regards