Whitelisting Gmail Emails

Emails from certain domains are aggressively identified by Gmail as spam, although being legitimate. I've been trying to train the spam filter by flagging the false positives using the "Not Spam" button. However, the efforts do not appear to be helping. It is possible to whitelist emails are not spam. To do this, create a filter for the email. One of the actions for the filter is "Never send it to Spam."