One of our managers is going on extended leave. He receives emails from internal staff and external clients. While on leave a staff member will be dealing with the client enquiries received via email, but not the enquiries from within our business.

How does the manager add a rule to forward all emails received from external clients (ie not on our domain) to the internal staff member, but not forward emails received from internal staff.

I can see how to create a rule to forward all emails, but not only those from external clients (who will all have different email addresses).

Thanks for your help