A few pithy thoughts from Jakob Nielson and useit.com and his 10/20/08 Alertbox on Transactional Email...
Good email that respects users' time and quickly tells them what they need to know can do wonders for your customer service reputation.
In general, you should write a message according to the users' priorities, starting with the information they're typically looking for.
..processing email is stressful. Users frequently told us that they were too busy to deal with certain email messages, and that they considered any fluff in messages a waste of time. When they check their email, users are typically dealing with multiple requests for their time — whether from their boss, colleagues, or family. People just want to be done with most email, and quickly move past anything that is not absolutely essential.
Jackob Nielsen also goes on to stress making subject lines relevant to users reduces the chances an email will get deleted in the process of pruning/deleting SPAM messages.