Mac mail and MS exchange - how to whitelist email?

I just started using this combination for email service. It works great except for one annoying thing. I have some legit emails that always end up in my exchange junk folder. I click the trash can item to "move selected items to inbox" and that works, but the next time that same sender ends up in junk again. There is no option to "always accept email from XXXX" or something like that. Any advice?