how to change the primary smtp address from upn to another value for entra users?

Hello,

We are currently in a project to use the M365 services.
The use case or question specifically concerns EntraID and Exchange Online.
In the Microsoft cosmos you get the UPN (UserPrincipalName) and this is usually also the primary SMTP address .
However, there are parts of our company that have their own mail address/domain.
We would like to change the primary SMTP address for these and store the address from UPN as an additional mail address.
Presumably there are other companies that have already done this, so what experiences have they had?
What needs to be configured in OneIM, as well as in the tenant, so that this adjustment can be made?

Thanks for any advice.

kind regards