If you don't have a mailing list and want to build one and make sales in the process, then use these emails for paid solo ads, adswaps, blog posts and social media content posts.

If you have an existing mailing list, but haven't emailed them for a while, then it's most like that they've forgetten who you are or why they signed up.

In which case it may be a good idea to send out these emails to promote your squeeze page to enrol them in your email ecourse which promotes your product.
