After building a hybrid Exchange environment as outlined in a couple of previous blog posts we have an Exchange 2013/2016 environment where some Mailboxes exist on-premises and some Mailboxes exist in Exchange Online. Autodiscover is still pointing to the on-premises environment, and so are the MX records. Inbound SMTP mail flow from the Internet is still accessing the on-premises Exchange 2016 Edge Transport servers before being delivered to the intended recipients.
Figure 1. The Exchange hybrid environment with Mailboxes on-premises and in Exchange online.