Office365/exchange online/outlook email with certificate

Hello,

I need some help from my trusted sources :)

One of our partners now requires that email communication be certificated.

Now, how can I do this with a office365 (exchange with outlook client) email account? Currently, I need for 3 users.

Since it's not a company requirement, for now, a solution for a user basis is fine. But how to do it well? Honestly, I never had the need to implement this.

I welcome any help :)

Best regards