[a_subsystem]

Our domain checks out because we use O365. However, we have an old Exchange server sending out via SMTP. We're not sure what the best path forward is for us. Do we change our apps to route through O365? Will probably take days for that. We have in house custom apps that use it heavily. Anyone have good resources on what we should do?

[dashgreen]

With the appropriate configuration, your SMTP server could relay to O365, just acting as a forwarder, you don't necessarily have to remove the server. This is very common in use-cases for old devices that barely support SMTP authentication, never mind TLS!

[technion]

There's an oss dikim signing transport plugin for on prem exchange that has become somewhat a standard in this space.

https://github.com/Pro/dkim-exchange/releases