Over the last couple of days, I have installed a Mailcow server in an Exchange Hybrid setup following this page https://mailcow.github.io/mailcow-dockerized-docs/third_party-exchange_onprem/ and others on the Internet.
Some email accounts will be on O365. Others on Mailcow. O365 is the front server (receiving all inbounds emails) and relaying them, if applicable, to Mailcow. Outbound emails from Mailcow are ALL relayed by O365.
I am not sure about the right thing to do with the DKIM key.
When I initially installed mailcow, I added a public TXT entry (dkim._domainkey) to the domain DNS with the DKIM key provided by Mailcow.
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 Mailcow as it is
2) delete the TXT entry with the DKIM key provided by Mailcow 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)
3) or keep 1) and add 2), meaning that there will be 3 entries for DKIM in the DNS (one from Mailcow and 2 from Microsoft)
4) something else
It would be great if you could advise me.