WhitMail uses Gmail's spam filtering system to keep junk messages out of your inbox. The spam folder on your WhitMail page is located at the bottom of the labels on the left side of your web browser. You may need to select More to see the label itself.
WhitMail automatically places suspicious mail in the Spam label, so you may want to check it occasionally for legitimate mail. For more information about WhitMail spam filters, take a look at Gmail's guide on using spam filters.