Cease & Desist Notice For Trademark, Copyright

One of my known competitors has sent me a notice to stop using his company keywords on my website where all the words are very common. However, he has the trademark registered for common words in a particular order which he feels is being wrongly used on my website as keywords to claim my brand name as theirs. In my opinion, there is no similarity between my brand and his apart from the three common words which I think are not copyrighted as they are commonly used words. I am not using his brand logo or the words claiming to be his company. My brand name is clearly differentiated itself and also I have a registered trademark of my firm. This is a pure case of professional jealousy and harassment to me without any legal standing. As far as I am concerned his trademark is to be canceled as they are completely common non-trademarkable words. I need guidance for the same and assistance in response to the notice sent to me.