There is no Safe sender list (or Whitelist) "out of the box" in Gmail. Whatever further emails from a particular addressee do not end up in spam, there are several methods that we will describe below.
Mark it as Not Spam (if the email is already in spam).
Create a filter
Settingsbutton in the menu that appears.
Filters and blocked addressestab.
Create a new filterbutton.
Fromfield with the desired email. If you want to add all emails of the domain to the white list, type "@domain" (example: "@example.com") in the "From" field. Leave the rest of the fields empty (so that other possible rules will not apply to our filter).
Never send it to Spam