I came across this problem while trying to forward a client’s email to several people at once. I looked up where to find the setting:

Preferences Tab -> Mail -> Recieving Messages section (just scroll down a little)

But all I saw was one single box, fit for only one forwarding address.

Upon searching through Google for a solution which would allow me to add as many addresses as I wanted, I finally talked to a coworker who told me that a simple semicolen ( ; ) would allow for multiple addresses to be inserted.

For example: (email@address1.com ; email@address2.com ; email@address3.org) etc.

It works perfectly! Just like within Outlook.