1> It is recommended that everybody compose and send mail in Plain-Text format. Why do you send e-mail in unnecessary HTML/Rich-Text format?
For M$ Outlook there is way to make your default composition option as plain-text. To do that go to Tools -> Options -> Mail Format and select "Plain Text" for compose message.... Drop-down list-box.
By doing above your NEW email composition will be into plain-text. If you are replying to an e-mail which is already into HTML then your Outlook will intelligently start composing into HTML only. So it should not bother to people who doing customer communication. Other way if you replying to a mail which is in plain-text then your Outlook will compose in plain-text only.

Why plain-text?
It consume less bandwidth.
It is compatible with all Mail-Clients.
Probably you are not part of some forum/mailing-list; many of them strictly prohibit HTML mail or they usually don't parse HTML tags and your message looks very jumbled.

2> Please use relevant subject. Don't use your-name, or regarding, or today's date, etc trivial value into subject line :-)

3> Provides as much details as you can while writing mails to
team-members. If you drafting mail to customer then usually strip-down info written.