Am trying to set up an outbound mail connector and rule within https://admin.exchange.microsoft.com/#/connectors to route messages FROM a particular user, out to a 3rd party delivery service such as SparkPost, Sendgrid etc.
These services accept SMTP mail on port 25 or 587, with TLS, and expect username and password credentials to be presented on SMTP connection establishment. Messages are relayed on (with DKIM signing etc) to the final recipient.
This help article describes similar, but it's oriented around delivering mail TO a particular user.
There doesn't seem to be a way for O365 to have a connector to services like this. The connector setup requires a mailbox identity to verify against; not a username and password.
Further, the limit on mail flow thru an O365 account looks to be 10,000 per day, which is a lot for a human user but small for apps generating mail.
Looks like they'll need to keep Exchange running on-prem to fulfil this use-case, unless anyone knows otherwise?
I spoke to Microsoft tech support and there appears to be no way to set up such a connector. Also they do discourage folks from using O365 for bulk mail, but sometimes grant enterprises exceptions.