We're often advised to keep our eMail messages to "one screenful" (whatever that is). What's more important, in our experience, is to limit the content of a message to the topic named on the Subject line. If you've got several things to say, put them in separate messages.
Success Strategies
If you want your eMail read, keep it crisp and to the point.

| Write a short, pithy Subject line, and
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| Stick to that subject in the message.
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Short, Pithy Subjects
To write a Subject line that encourages people to read your eMail is an acquired skill. The first rule is figure out what would inspire them to respond; it doesn't matter what you want. The Subject line that reads "Help!" marks the sender as a novice. Experienced users usually just skip messages like this, because they've learned it takes too much time to open it, try to understand it, then discover the subject isn't one on which they can offer advice.
The Subject line that says, "Help Me Solve This Microsoft Word Problem" is a little better, but it describes your problem, not the help you seek. A Subject line that says, "Do You Know How To Build An Index In Word?" is more likely to elicit a response from someone with real experience. Make it easy for a recipient to want to share information with you.
Stick To The Subject
If you don't stick to your topic, you're guilty of "bait-and-switch," getting people to read your message with one Subject line, then switching to a different topic inside. That's a favorite trick of people who send you junk eMail ("spam"), and you'll get the same reputation if you do this very often.
Once you're in a dialog with another party or group, you'll all tend to keep the same Subject line. If your eMail program is smart, it puts a "Re:" in front of the Subject, so other people know it's a response. Make sure you stay "on message" when you respond. If you add new topics, or change to another topic under that Subject, you may confuse people. Worse, your new message may never get read, because people weren't interested in that particular Subject line, and deleted it without reading.
Some people lapse into a meandering, unfocused way of expressing themselves in eMail. Their readers are then burdened with figuring out what to respond to. When you're faced with a mountain of eMail to wade through, which messages are you likely to respond to: Those that are crisp and precise, or those that are rambling and disconnected?
Put yourself behind your reader's eyes. Make it easy for them to respond to you.
Sometimes long, multi-topic messages are appropriate: If you're reconnecting with a long-lost friend you might relate some family and work and hobby information all at once. However, if you expect a specific response to one particular part of your message, you're more likely to get the response you want by making that a separate message.
Responding To Multi-topic Mail
Not every eMail correspondent will have become as sophisticated as you. What can you do with a long, rambling message?

| Respond only to that part of the message that is related to the Subject line; that sends the message that it's not appropriate to load other subjects into a message.
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| Respond with several separate messages creating a new Subject line for each.
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| Don't respond at all. When they ask why you didn't respond, you can safely reply, "There was just too much for me to wade through; I couldn't figure out how to meaningfully respond." Again, it sends a message that long, ill-formed eMail messages aren't going to be rewarded.
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