One domain with several mx records, allowing some users to use Google Workspace and others to use roundcube from the server

All setup and MX records have been configured for the domain example.com, which I added to my Google Workspace account. I also own the website example2.com. These two domains belong to two separate businesses with various users. In the case of example.com, all users are required to have a Google Workspace account and a Gmail address, such as [email protected], whereas, in the case of example2.com, some users are not required to have a Google Workspace account. Of course, the two businesses have two distinct populations of users.

What I need to do is configure the second domain, example2.com, so that if a user has a Google Workspace account, they can send and receive emails using their Google Workspace account and mail, but if they don't, they can still use the default mail server to send and receive emails from the server, not Google Workspace.

I consider creating several MX records so that everyone with an email address and user for example2.com in Google Workspace can utilize Gmail, Google Workspace features, and other services. However, if someone only has a standard email set up for them in the domain's mail server rather than a Google Workspace account, they can still use that email. I'm referring to setting domain mail server MX records as the second priority after Google MX records.

Multiple MX records does not make all the MX servers receive a copy of the message, but only the first one available in the preferred order will get it; that is the one with the lowest number in the priority field. Whether it is Gmail or an own SMTP server, it has no clue what to do with messages sent to non-existing users from its own perspective.

In your mail infrastructure, you need MX servers that can accept inbound mail for all the users and then direct it to the correct mail storage (there are many strategies for this). Only outbound mail can have multiple sources directly.