Let's think about it, sending an email and having the receiver to transform is a little bit like going out in a club to score.
Let me just call my email John.
John wants to score. He is in his apartment downtown and is dressing up. If he wants to score he probably has to dress as well as possible to be the more attractive possible. This is the creative.
Once ready he heads to the club, this is the broadcast.
If he arrives there without any incident (delivery) he gets in the line outside the club and wait for his turn.
Then he gets to go through the bouncer (the spam filter) who will let him go in if:
1 - he matches the dress code (Tidy HTML)
2 - he never went in a fight in the club or got thrown out (Reputation)
3 - He isn't drunk and talking crap (content scan)
When he gets in (inbox) he spots a good looking girl he likes if he already knows her, she's a client, else, she's a prospect.
He gets close to her and then throw his pick up line (subject line). There are several types of pick up lines:
1 - If you lie about yourself, it's a scam
2 - If you are too mysterious, you might loose her attention
3 - If yo are too long you might be boring.
In a few words, your subject line needs to be as good as your pick up lines: short, punchy and interesting.
If the girl gets in the discussion with John, then he got a click, but there's a lot more to do to score.
He has to try to keep the discussion live and interesting and to gather as much information about the girl so he knows what she likes, what she wants,... that's web analytics.
The interesting thing here is that even if he doesn't score right away with that girl, the more information he will get here, the more likely he is to score on a later night when they get to meet again.
As I see it, email marketing is like trying to score with hundreds, thousands of persons in one go.