I just ran through the Exchange Hybrid configuration wizard yesterday as we want to move some users to Office 365.
I am currently running through my testing and have found that the user who's mailbox exists in O365 can receive email (both from internal and external email addresses) but they cannot send to either internal or external addresses.
I don't get any non delivery reports or anything like that, the emails just fail to arrive at the recipients mailbox.
I am presuming this is a send connector issue but I am struggling to find the information I need. Can anyone help point me in the right direction?