Teams emails not being delivered to internal users

I have been doing some testing to implement teams. It seems like notification emails are not being delivered to our internal users. External users (hotmail.com) are being notified. We are not using exchange online, but our own on-premise email system Zimbra. I am wondering if there is a routing issue with the exchange online system, event hough we don't use it. I am not sure if this is a Teams or Exchange issue.

Spencer