Pay attention to punctuation, spelling, grammar and capitals. It shows that you value us and that you've thought about what you've written rather than an off-the-cuff rushed message in the heat of the moment.

Your subject line should be descriptive, especially when we get so many emails or if we like to save the emails in a file. It'd be great a practice to start the subject with “Recipient or group name” then a hyphen, your subject (specific and changed for each email), another hyphen and then the date.
E.g. Thea–Email tips attached–15Oct05 Make the subject crystal clear.

Use short paragraphs and leave lines between them. This makes for more easy and quick reading when you experience a lot of email activity on a regular basis.

Check the source of any "news" or "chain" mailings before passing on.

Avoid sending unsolicited, large attachments. That 3MB movie file may be the funniest thing you've seen for a long time, but don't automatically send it to everyone to know. Ask them first if they want to receive it.

Ensure that your PC is protected against viruses. Your virus scanner might not protect against Spyware and Adware.
Use lower case font. When in all capitals, it is harder to read and may be perceived as aggressive.

Thou shalt not spam!
Re-read your message before sending. Read it from the perspective of the recipient.

Consider first if your message needs the “Reply To All” treatment, or if it is sufficient to simply “Reply To Sender” only. Always ask permission if wanting to pass another's contact details forward. If you need to keep another's email details hidden when you “Cc:” then place their email address in the “Bcc:” text box.

Limit your “non-group related topic” emails to e.g. one per week, when using the group email option.
Keep emails short i.e. to a single screen page. When it's a more complicated issue to discuss, why not use the telephone and speak? In emails to busy people, tell recipients if/when you do/don't want a reply.

Be mindful of when you use the ‘priority' or the ‘request receipt' options (these are under ‘Tools' and ‘Message' when you have email open & ready to send). Less use has greater impact when you need it.

Is it clear who the sender is? Use an email signature that has contact details and change the “From:” option for your emails. To change what shows in the “From:” box, go to Tools, Accounts, Properties and change “Your Name:” in User Information. It won't impact account settings, it's safe to do.