If you don’t know the conversion rates on your optin page, then setup split testing. I recommend Leadpages for setting up your optin pages because they have built-in click tracking, A-B split testing, and they let you drive unlimited traffic. Of course, they have some beautiful optin page templates that you can edit very easily using their click-click interface.
The subject line should link seamlessly to your main headline and then to the lead copy and into the body of the email. Ensure that the subject line is relevant to the content and incites a relevant expectation – clickbait is bad, folks! Open rates are not the only metrics by which to measure success (I’ve heard it said that open rates are ‘vanity’ and clickthroughs are ‘sanity’).
Purchased lists are ineffective, and they impact everyone else who uses Mailchimp, too. If you send emails to a list of people whose contact info you bought, many of the emails will get identified as spam. Some spam filters will flag a campaign if anyone with the same IP has sent spam in the past. When you use Mailchimp, your email is delivered through our servers, so if one person sends spam, it could prevent other users’ emails from reaching inboxes. But by forbidding Mailchimp users from using purchased lists, we increase deliverability for everyone.

When you're using email as a marketing tool, you're working on a pretty personal basis. This isn't ‘drive-by' stuff -you're directly targeting customers from their inboxes. There's a huge amount of potential which comes with this kind of marketing... but you'll only reap that potential if you REALLY understand both the marketplace you're operating from, and the audience you're talking to.