on premise account doesn't receive appointment invitation / attendees response email

My main environment is on the cloud , office 365 , but we also have on premise.

When UserA (on premise acc)  invites UserB (not on premise acc) to a new appointment, and then UserB accept/reject the invitee

,UserA  doesn't get any emails responses. Also , when UserB invites UserA  to a new appointment , UserA doesn't receive any email.

any idea why ? 

thanks