1. Use sentence case. Typing your emails totally in lower case can create the perception of lack of education or laziness. Use sentence case to appear more professional (in other words use a capital for the first word in the sentence or any proper nouns e.g. place names).
2. USING ALL CAPITAL LETTERS or BOLD LOOKS AS IF YOU ARE SHOUTING!!!
3. Address your contact with the appropriate level of formality. Make sure you spell their name properly.
4. Be formal, but not sloppy. Commonly accepted abbreviations may be suitable for internal communications but, with clients or customers, everyone should follow standard protocol. Email messages reflect you and your company, so traditional spelling, grammar and punctuation rules apply.
5. Avoid High Priority and URGENT!!! If it's really important then use the phone.
6. Limit use of Read Receipts. They're intrusive, annoying and easily declined.