You can whitelist email senders in Gmail in two different ways. First, check your Spam folder. If you see an email there from Rainbow Plant Life, you will want to add it to your whitelist.
Highlight the email and click on the "Not Spam" button. Future emails from Rainbow Plant Life should now automatically go to your inbox.
You can use the second option if you haven't yet received an email from Rainbow Plant Life to whitelist.
You can create a filter to automatically send emails from certain domains to your inbox.