How to prevent Gmail from labeling your colleagues' emails as SPAM

You have to do this for individual colleagues one by one.

1. Go to your an email whose sender is among the one you want to add to the whitelist
2. click the little triangle on top right of that email
3. choose "filter messages like this"
4. In the new window, click "create filter using this search"
5. In the new window, check the box for "never send to SPAM"