How have you aquired this 15,000 addys. If they have been harvested at random, and they have not developed any form of customer relationship, or opted in to a mailing list then your emailing them, en mass is undoubtedly spam. I strongly suspect you would be breaking the law.
You could easily find your ISP getting tough with you.
http://www.dti.gov.uk/industries/eco..._200258ec.html