Question:a. include all pertinent topics to avoid sending many separate messages.
b. convey a large part of its message through the use of abbreviations and emoticons.
c. include an inside address and a businesslike salutation and complimentary close, just like a paper letter.
d. preferably be about one topic, which should be summed up in the subject line.
All of them i guess! Depends on the situation and the recipient.
'a' is fine if you are posting a community message
'b' when you are casually chatting with friends
'c' if its an official e-mail or a mail which needs to be printed out