SMTP Relay after Migrating to Exchange Online

Many customers we onboarded leave a down-sized environment of current Exchange servers in Hybrid for recipient management and mail relay. Depending on your requirements, this may be the only supported solution as throwing 3rd party gateways in the mix for relaying results in application mail, MFC scan-to-mail etc. getting treated like regular mail, having it ending up in junk-mail or worse for internal recipients, causing you having to implement bypasses to prevent this behavior.

@Julian Benson

You could follow this article to uninstall the Exchange on-premises if you don't need to use Exchange server, AAD Connect and ADFS. If you still use AAD Connect and ADFS, you need to keep at least one Exchange on-premises server (Scenario two in that article).

About the relay function, you could use Exchange online to relay emails for your APP. Personally, I'd suggest you keep Exchange on-premises to relay mail, because it is easier to configure to relay than Exchange online.

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Normally you need a licensed user to be able to send e-mails using SMTP with Office 365. Your applications also need support for TLS encryption. If your application doesn’t support TLS, or if you need to send emails from another address than the licensed user’s address, you need another solution. Luckily Office 365 can help you. The solution is to set up an inbound connector in Exchange Online Protection.