Sending email to a past client of my past company?
At this point, enough time has passed in my mind to make this ok.
The only thing you need to verify first is that you did not sign some sort of document protecting your previous company against you doing what you are considering -- contacting their client.
Once you have determined you are legally ok to proceed, take your time and construct a nice email, or better yet if you have a contact at the company, speak to someone directly.
NOTE: IANAL, and the "legalness" of this will vary.