E-Mail is Personal
E-Mail, whether formal or informal, is a very personal medium similar to personal conversation. If you send an E-Mail to someone and he/she does not respond, you will feel anxious and disappointed. Use auto-acknowledgement and out-of-office replies appropriately to let people know you received their E-Mail communication. If you are using auto-responders, avoid E-Mail loops by using varying "from" addresses. Use clear signature names at the footers so everyone knows exactly who is writing to them and how to contact the sender.
In an e-mail campaign, use personalization features for your mailing list in the program you use. "Dear John" sounds more intimate that "Dear Customer". Sometimes, it may mean the difference between the recipient reading your e-mail rather than consigning it to Trash.