I have a PHP script that sends out email from a non-Office 365 SMTP to Office 365 accounts. Messages get delivered successfully but when I send email to a non-existent email accounts on a domain that is on Office 365, I dont get any bounce emails. I have no problem getting bounce emails from Gmail or other domain that is not on Office 365 though.
Anyone got an idea of why this would be? Am I missing a header?
This depends on the connector settings of the Office 365 tenant you are sending messages to. Office 365 allows mail admins to route messages with certain criteria to an internal SMTP service or a smart host. For those messages, Office 365 may think they have been routed to another server successfully. Actually the messages might be lost by a smart host, or the smart host doesn't support to return NDR. You can try to trace the email in Office 365 to find the routing path. Meanwhile, check the connectors, transport rules configured in the corresponding Exchange Online admin center.
