A - Address & AttachmentsCheck the email id before replying or else you may either have sent the message to someone else or will be embarrassed by a Delivery Failure Response. Also, avoid sending business emails from email ids such as funnyboyz0018@....com. It creates the impression of you being unprofessional. Check the attached files before sending them. Also, if you have mentioned in the mail that you are attaching a file, check if the file has been attached before hitting the "Send" button.
B - Brevity & BandwidthTry to keep business mails as brief as possible. Emails are best when they are brief and stick to that one singular subject filled out in the box called "Subject". Respect other people's bandwidth. Compress or resize large files if you need to send them.
C - Clarity & CCClearly articulate what you wish to communicate. An abstruse message not only wastes time but also confounds, if not irritate. The CC option is FYI (For Your Information) copy and the email ids submitted there are seen by all. The BCC emails are hidden from all. Use them as needed and properly.
D - Deliberation"Think twice before you speak once" also applies to email communication, especially. Don't just write anything just because you think you need to write something.
E- EmoticonsTry avoiding emoticons in business and formal writing. Emoticons and smileys may only be proper for personal emails and must never be used in business writing.
F- ForwardingBe careful when forwarding an email to someone else. You need to be sure the writer would not be offended by it. Also, only forward mails if you think it is necessary and try to explain with a little note to the recipient why you think this forward is important. Remember that "forwarding" doesn't rate up as "communicating" and can never be a substitute for the latter. Better, don't forward emails very much. Especially, don't forward emails that may be spam or hoax emails. Try to check them out at snopes.com, preferably first.
G- Greeting, Concluding, and Grammar/SpellingGreet properly. Don't just drop in suddenly without any proper salutation. Know when and how to use various forms of greeting and use them properly, to the occasion. Be polite. Also, end the email with a proper concluding remark; for instance, "Warm Regards," or "Sincerely". If you're not sure how to close the email with, then simply close it with the word "Thanks".
Sms and chatting habits easily creep into email writing and damage formal writing skills. Check your grammar before you hit the "Send" button. A bad grammar could render a message either unintelligible or miscommunicated. Also, check your spelling. You may remember the story of a secretary who mistyped "affect" for "effect" and so caused such a great loss to her company that her boss was fired for signing the misspelled letter.
H- Helmet-Emailing or FlamingDon't use emails as shields behind which you can hide while flaming others. Write keeping the person as person before you in mind. Write as if communicating in person. Don't just write something off and then hit the "Send" button thinking you're safe behind the computer screen.
I- InformationMake sure you have included all necessary information necessary for your message to be properly understood and be properly responded to or acted upon. Type complete and meaningful sentences.
J- JokesEmail may not be the right place for communicating tongue-in-cheek humor. Also, don't insert a joke where it might be read not as a joke but as something serious. Avoid sarcasms.
K- Keep It Short And SimpleAvoid long sentences. Also, avoid long mails.
L- LanguageForgive the language of someone who is kind enough to reply but doesn't have English as his/her first language. Try to understand as you wish to be understood.
M- MoodNever send an email when you are in a bad mood or are upset.
N- NicetyObserve nicety in your email communication. Use preciseness, politeness, and propriety.
O- OmissionsIt is advisable to mention why you are omitting response to a point when you omit it in a reply, and also stating if you think you wish to respond to it later. Responding but omitting a point might indicate that either you didn't consider the point worth a response or you were too careless about reading this particular mail.
P- PriorityAddress priority points first. Respond to the most important things first.
Q- QueriesCheck if you have responded to every query, or at least, mentioned that you wish not to respond (if not why).
R- ReplyRead the email properly before you decide to reply. Reply to the mails that anticipate a reply and that you wish to reply to as quickly as possible. Don't procrastinate; otherwise, either you may forget or the reply might become too belated. Also, reply to each query or point in the email properly and clearly. Don't assume that you don't need to reply to a mail, unless you plan to discourage the sender from writing to you. Also, make sure that you really need to do it when you choose "Reply all". Also, you don't need to reply to every email, especially spam.
S- Subject Line & SignatureTry to keep one mail per subject so that it is easier for the recipient to respond to each topic separately. Try to keep the subject line as clear as possible, so that part of what you want to say is already communicated in the subject line. Your Signature must be proper and not include unnecessary details.
T- Time and TimezoneRemember the location/time-zone of the recipient; their business hours may come later than yours.
U- Upper CaseDon't use all upper case letters. Upper case letters must only be used for emphasis. Using all caps indicates rashness, hurriedness, or even YELLING and is considered bad etiquette. Similarly, using only all lower cases (small letters) gives the impression that either you are lazy or you are not well literate.
V- Vanity CheckDon't decorate emails with vain formatting, colors, and stylish fonts. They create the impression of vanity.
W- Walls/BoundariesMind your boundaries. Respect boundaries that are yours and also boundaries of others. The very concept of communication involves an understanding of boundaries.
X- X-FactorNot only reply to the questions asked by the sender, but also reply to questions that the sender is likely to ask in the next mail. Be proactive. Anticipate. Go beyond the box as necessary.
Y- Yet-To-ReplyMark as unread mails that you wish to reply later. If you wish to reply very late, then it is proper to send a short message that you have read the mail and perused its content and would need some time before you can reply. Simple silence (no reply at all) might indicate to the sender that either his/her mail had offended you or you didn't consider his/her mail important enough to deserve a timely response.
Z- Zoning OutDon't reply if you're zoning out, dozing off, or feeling too tired and exhausted.
NOTE: The list above is not conclusive.