Full Hybrid - add accepted domain STEPS

Question

Hi ,
i have a question about a full hybrid setup and a new accepted domain.
We need to add an extra accepted domain.
We need to add it on O365 , that is for sure but we also need to add it on the on prem server.
External mail flow is still going from our mail relay ==> onprem ==> thru connector to O365 .
Do i need to rerun the hybrid wizard ? Or what are the steps ?

Answer 1

No need to re-run the Wizard.
Depending on how your connectors are setup for the address spaces, simply add the new on-prem one to the 365 > on-Prem connector and you are good.
IF the existing connector already is set to route all accepted domains to on-prem then you you do not have to make any changes there at all.

If this new domain is set as a primary SMTP address for some users and those mailboxes are on-prem, then add the new domain to the organizational sharing
https://learn.microsoft.com/en-us/exchange/sharing/organization-relationships/modify-an-organization-relationship#use-the-exchange-admin-center-to-add-a-domain-to-an-organization-relationship

Answer 2

At this point , all mailboxes are in the cloud and the new accepted domain is needed because we need to add aliases to the primary smtp addresses.
So there shouldn't be any messages sent from O365 to onprem, as they are all in o365.
We are not at the stage yet to switch DNS and MX.
but , still not 100% clear for me.
I add the accepted domain on O365.
Do i need to add it onprem ?
The onprem connector will route xxx.mail.onmicrosoft.com to O365 ...
i dont have to adjust the O365 connectors then ?

Answer 3

i assume , that you need to add this domain to the organization relationship as these mailboxes are still on prem and you require free/busy status ?
As i understand it correctly the mail will be routed because that mail enabled user has a primary email address and the new alias but the targetaddress is xxx.mail.onmicrosoft.com so it will get routed correctly to O365 ?