should you mention what your salary expectations are?
If you are contacting an agency who will work on your behalf to help you find a job, then Yes - you need to tell them what salary you would be willing to accept. Otherwise, they will not be able to find jobs that are a good match.
If instead you are contacting a recruiter who is working for a company, but it's not about a particular job, then I assume you just want this recruiter to "keep you in mind". In that case, there probably is no advantage to stating your salary requirements. Wait until a specific job appears, or until you are specifically asked for your salary requirements.