Microsoft graph and getting Office 365 mailboxes

hi everyone,

We have a home developed identity management system built on .net, and we want to be able to connect to Azure AD, and get basic information  about each user.

as we have users with mailboxes on premise and users with mailboxes in Office 365, we want to connect to Azure or Office 365 from our .net application, and see if that user has mailbox on Office 365. It is not enough to learn his email address. We actually need to know if he has a mailbox in Office 365 as knowing he has an smtp address might mean that he is on premise mailbox user.

I am looking at the Microsoft Graph documentations, and I am kind of lost here, how can I tell from the Microsoft graph that a use has actually a mailbox in Office 365?