What does "Suspension Letter Mailed" as a registration status mean?

A suspension letter will be sent to an applicant where another preexisting, pending Section 1(b) intent-to-use application is cited as potentially creating a Section 2(d) likelihood of confusion problem. Prosecution of the applicant's application will be suspended pending resolution of the intent-to-use application. This can be a frustrating situation for the applicant as it may be stuck waiting, literally, years while the intent-to-use application files repeated Requests for Extensions to file a Statement of Use.

In short, barring other facts, the trademark application you found is still alive and the trademark itself may be in use regardless. Your ability to use your desired trademark will also depend on the classes of services and goods to-be-associated-therewith, as well as other preexisting applications or registrations. You should consult with a trademark attorney before advancing too much further with your branding strategy.