Many of the same considerations hold true for writing an effective email post as writing an effective webpage. Once you have laid out what you want to say, step back and look at it from the viewpoint of a reader -- one who does not already know and agree with everything you are saying.
What is this about? If you are replying to a post, include the name of the person you are replying to and enough quoted material from their post that a reader coming in cold can track the discussion. And no more, please! Quoting 100 l ines to add "I agree" will annoy a great many people, including those who have to pay for their internet access time by the minute -- and eventually, if you make it a habit to include pages of irrelevant quoted material, they may stop reading any of your posts.
If you are writing a new post, make the subject line clear. "Alarming news!" may get your post opened, but probably only the first hundred or so times you use it.
Incidentally, if the subject of a discussion splits off on a tangent -- which is usual on group lists -- please change the subject line, so that people do not continue to open "house construction", which they were originally tracking, to find a discussion of Middle Age English grammar.
This may be too much to ask for, but do try to be open about the purpose of your post. If you claim to be providing background information to a current discussion, but you are actually trying to get a rise out of your old enemy Beeblebrox, or you claim to be introducing useful resource information but you are actually fundraising, you may be effective but you are also going to wear thin with many readers and eventually find yourself filtered out of their inboxes.
Be concise. This also may be too much too ask -- especially since it is me who is asking it, and those who know me may very well be muttering right now, "Those who can, do ..." and "Practice what you preach, kid."
On that thought, I am going to end this page. For now.