Will the Microsoft License email for a staff member override Board member emails in different domain

My organization's Board of Directors uses Vieth (CPanel) for our email domain and server. We are hiring a new Executive Director and want to purchase Microsofit 365 for nonprofits to provide her with everything to be successful, including the same email domain as the Board. I've heard that if I switch the email domain into Microsoft, then no emails will go into the original email provider. Does this only happen to the individual email that was moved over to Microsoft, or to all of them?

Our ideal situation:

- email address removed for privacy reasons is on Microsoft 365 with Outlook and sends/receives emails

- email address removed for privacy reasons is on Vieth (CPanel) and sends/receives emails

I want to avoid a situation the discontinues the ability for our Board members to continue using the original domain if it is overridden by the Microsoft one.

I hope this was clear! I appreciate and reassurance or guidance. Thank you in advance!