tl:dr – Most people will ignore your e-mails. You can't change that. Limit the content and provide simple summaries in the first subject line and first sentence.
The first sentence in this e-mail from UserConf made me chuckle (open to read):
UserConf understand the problem. No matter what you do, most of your audience won't read your e-mails. Even for the most important announcements, most people won't read it.
You could literally start your e-mail with the subject line: "Super important announcement that you must read!" and people won't read it.
In our inboxes we scan the subject lines, possibly the first sentence, and that's it.
This also tells you the solution. Put the information in the subject line and first sentence.
You can provide the rationale later. Just get the message condensed into those first two lines.
p.s. Good post on notification design strategies on UXBooth.