It really depends on how these leads were acquired and how targeted they are (ie are they in a specific niche, or just random email contacts?). You should first start by making sure these are real, working e-mails. If you have too many bounces or low quality leads and do cold emailing wrong, you can end up in spam folder most of the time, reducing your conversion chances. You can use tools like Neverbounce to do that. After, you should create and manage mailing campaigns with tools like Mailshake, for example.
For us to help with the products or services to promote it's really going to depend on how targeted they are.