Trademarks: Cease and Desist Notice for Use of a Business Name

I have been operating as a DBA for the past 14 years under Gotham Financial Services. Recently I received a letter from an attorney stating that his client (Gotham Asset Management) is the owner of federal trademark registrations covering "financial services". He claims that his name has acquired substantial goodwill and consumer recognition in the marketplace and is asking me to cease using my DBA name. Most of my business centers around insurance with some in investments. The other firm is MUCH larger and frankly we do not overlap in target markets or client base. Am I not allowed to use the word Gotham in my name? There are numerous companies with the word Gotham in their name. What recourse or push back do I have?

Trademarks: Cease and Desist Notice for Use of a Business Name