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’).
The terms ‘aim' and ‘objective' are often used interchangeably. However, they are not the same. Aims are an overarching end goal (and are usually specific). Objectives are steps along the road to that goal (and can be a bit more flexible). Aims are long-term outcomes, while objectives are short-term targets. Different objectives work towards different purposes. For example:
Everyone's busy and their inbox is already full. Why add to the problem with a longwinded email? People generally like short, concise emails better than long ones because concise emails have an obvious focus. Plus, when your users are scanning through all their emails in a short amount of time, they're more likely to find the overall message before deciding to take any action.