So....

I can't find a definitive answer one way or the other. Can on-premise mailboxes be accessed from Office 365 in a hybrid environment?

Everything else seems to be working fine but if a user with an on premise mailbox logs into Office 365 and clicks the Outlook icon it gives an error. Users with mailboxes at Office365 work as expected.

Am I missing something?

If a user with an O365 mailbox logs in to Exchange OWA, they should be redirected to O365. In the reverse, you should get an error saying you have no mailbox in O365.

So you are saying there is not a way to access on premise mailboxes from O365? Does that seem half-baked to anyone else or is it just me? That sure would be a great feature.