You can still have SMTP configured in IIS of 2019 server and then have a connector in exchange onlinen to accept those emails from the onpremise relay. You can also check the Option 2 from the Microsoft Article https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365
Software compatible with SMTP Relay with Modern Auth
After reviewing a lot of info about Basic Auth, and when Microsoft will disable basic auth, is there any paid solution (secure and reliable) to connect multifunction devices with Exchange online with a shared account impersonating users? Now that IIS 6.0 and SMTP role is no longer receiving updates. Having in mind that there are a few of scanners and printers that do not support OAuth2 or MFA. I appreciate any advice.