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’).
Personas provide a multi-dimensional method of targeting. They’re based, as the name suggests, on a projected persona for each customer ‘type’. Get it right, and personas can help you enormously in both predicting behaviour and personalising your communications. Personas are a powerful technique, and they’re increasingly used to improve the usability and customer centricity of communications.

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.