Trademarks: Does the Other Guy Have a Valid Trademark

I recently recieved a C&D letter from another photographer in my area for using his "trademark". He started using the term to describe the photography product/service a couple years before I went into business, but he only started marking it with the "TM" symbol and registered with the USPTO last month. The term is very generic and descriptive and I don't believe it's a valid trademark. The first word is the thing in the sky behind the people in the photograph, and the second word is "portraits". Similar to "sunset portraits" or "milky way portraits". I saw it as a mere description, and I do a similar style of portraits, so I used the same term. It would be extremely difficult to describe my services without using this term. I don't think the term will hold up in court, but I want to be as sure as possible before I invite a suit. My biggest concern is that he was around first and the only one in the area doing this type of service until I came along. Is that enough?

Trademarks: Does the Other Guy Have a Valid Trademark