We have an Exchange Hybrid setup: one server on our premises and one office 365. O365 is the front server (receiving all inbounds emails) and relaying them, if applicable, to the on-premise server. Outbound emails from our server are ALL relayed by O365 to external recipients.
I am not sure about the right thing to do with the DKIM key.
When we initially installed our server, we added a public TXT entry (dkim._domainkey) to the domain DNS with the DKIM key provided by our server.
But after the Exchange Hybrid is now setup (with Split Domain Routing) I wonder what I should do:
1) keep the initial TXT entry with the DKIM key provided by our server as it is
2) delete the TXT entry with the DKIM key provided by our server and add O365 DKIM keys (done by adding two additional CNAME entires according to that page https://docs.mailshake.com/article/222-dns-record-microsoft). Also, deactivate DKIM marking by our server (as it would be entirely handled by O365)
3) or keep 1) and add 2), meaning that there will be 3 entries for DKIM in the DNS (one from our server and 2 from Microsoft)
4) something else
This page https://docs.microsoft.com/en-us/answers/questions/117045/office365-dkim-and-email-relay-server.html tends to make me think the answer is 2) but unsure
It would be great if someone could advise me.