Usign OWA, OAB, Active Sync when primary DAG servers switches to secondary

I have 2 servers set up in a simple DAG setup with my one witness server. Mail1 - Primary, Mail2 - Secondary, and FS1 - Witness server. 

Of course on the first server mail1 all of the external settings are populated for OWA, OAB, and so on and works fine. So what is the proper way to have external OWA, OAB, and so work also on mail2?  Lets say when I take the mail1 offline for or when the primary database moves to the 2nd server mail2?