I'm having an issue migrating exchange 2010 to 2016.
I'm on the phase that had exchange 2016 setup and coexists with 2010, but when I try to send emails from a new mailbox in OWA, it failed both internally and externally. the emails will just sit in draft. the new mailbox does receive emails either.
so far I only created an internal relay connector and everything else is default.
Try creating the send connector, if you already did that changes: http://www.mustbegeek.com/configure-send-connector-in-exchange-2016/