Over the last few years, I've been migrating Domino/Notes mailboxes first to Exchange 2016 and then to O365. Despite all the mailboxes being migrated there are numerous Domino applications still in use.

Currently, the co-existence works between the on-prem Exchange 2016 servers and Domino, with all mail flow to and from O365 via the Exchange servers. The routing addresses used are user@domino.domain.local and user@exchange.domain.local, which are not compatible with O365.

To remove and shut down the legacy Exchange servers, I need to route traffic directly between the Domino servers and O365. While mail flow is almost entirely one-way from Domino to O365, I still need to keep both directions working.

When I looked into this there wasn't much information specifically mentioning Domino, but there is information on routing connectors in O365. The steps are:

Get an external fixed IP for Domino. This will be hosted in the Azure Landing Zone so it will be traffic managed for redundancy.

Create a send and receive connector in O365, secure them by use of a 3rd party certificate - domino.domain.com .

Add a DNS record for domino.domain.com and point it to the external IP from Azure.

While a shared namespace is a possibility, for sending I decided to go for a child domain, so from O365 to Domino will have addresses of user@domino.domain.com .

Received mails from Domino can still use the user@domain.com address.

Once in place, all this requires is for the 3rd party cert to be renewed annually.

So far this is the plan, though there may be changes in practice.