First, contact an attorney who specializes in this area of business and law.
Meanwhile, also look up that law firm that sent you the letter and see what you can find out about them (e.g., are they even legit, what kind of other cases do they handle, etc).
And search for any information you can find about this other company and the people. What have they done? do they own property? What other businesses? Criminal records? etc. etc. etc.
Also gather all your own records - business formation, original site registration and registration history, logs of any marketing and advertising you have ever done, etc.. Anything that shows you actively using this trademark over time and as far back in time as you can establish.
Meanwhile, also look up that law firm that sent you the letter and see what you can find out about them (e.g., are they even legit, what kind of other cases do they handle, etc).
And search for any information you can find about this other company and the people. What have they done? do they own property? What other businesses? Criminal records? etc. etc. etc.
Also gather all your own records - business formation, original site registration and registration history, logs of any marketing and advertising you have ever done, etc.. Anything that shows you actively using this trademark over time and as far back in time as you can establish.
Bring this data to your new specialist attny.