Use plain text (and optimally a maximum of 72 characters per line).
If the message is not in plain text, will be reformatted.
If it cannot be reformatted, will be summarily rejected.
Include a useful Subject line.
Messages with an empty Subject are easier to miss
when someone is receiving lots of mails.
YOU may know what the message is regarding,
the rest of us who get several hundred messages a day have no idea.
Trim your signature.
Keep the signature lines at the bottom of your mail to a reasonable length.
PGP signatures and those automatic address cards are merely annoying
and normally stripped out.
Legal disclaimers and advisories are also very annoying.
Include important information.
If you have a problem,
do not waste other’s time with a hopelessly incomplete question.
No one other than you has the information needed to resolve your problem,
it is better to provide more information than needed than not enough detail.
Do your homework before asking questions.
I would not want to deprive you of a valuable learning experience,
and no one wants to see the same browsable questions asked over and over.