Email etiquette, and more broadly writing words for a web audience (from professional online copywriting to posting status updates for all to see on Facebook), there are tacit rules. Just like you shouldn't stare at strangers, you shouldn't bombard others from messages; and just like you shouldn't shout in a public place, you shouldn't write emails in all-caps. This email etiquette tutorial deals with capitalization across the board, including internet acronyms we all use as shortcuts in casual settings.
Typing an entire email message in lowercase gives an impression of sloppiness, and is harder to read (especially if punctuation is lacking or missing). Always use capital letters for the first letter of each sentence, including salutation at the top of the email message (if any). Acronyms like "FYI" ("for your information") should always be capitalized in full (all letters in uppercase). All-lowercase communication can be considered "OK" when it is a text message sent from a cell phone - even so, capitalize properly for important or formal exchanges.
Tip: there are some exceptions, like the word "iTunes", "iPod", or company names whose purposefully unusual capitalization is part of the branding process, whose first letter should not be capitalized even if it starts a sentence, and whose second letter should always be an uppercase regardless of context.
The stigma of "all caps", having a message typed in uppercase letters only, comes from the old days of the internet, before it became such a popular hit; but this convention has stuck: only use all-caps when you mean to shout what you are saying, and show that you are angry. We recommend against doing this, as it will only diminish goodwill from others, but this is how an all-uppercase message will likely be interpreted.
Caution: ALL CAPS is equivalent to "yelling", so always avoid!
Using uppercase for emphasis is quite acceptable at times, and will in fact unmistakably show to the other party that you truly appreciated their help. "THANK YOU!!" will have a sincere ring to it, and much more impact than the sadly common "Thanks", with or without exclamation point, which has nearly lost all its meaning (we have even seen some people make "Thanks" part of their signature, which in time means that they end up thanking you all the time for nothing, or sometimes thank you twice: one "Thank you" typed by hand, followed by the meaningless "Thanks!" in their signature - for more on that topic, please see our "Email Signature Etiquette" tutorial).
Composing an entire email message in bold font will most definitely come out as pushy, if not aggressive; but boldface is in some cases a nice alternative to using . To a lesser extent, using italic will do the trick as well. Avoid using colors to attract attention in a non-newsletter email; if you want a color, use a medium green, which is neutral (red can threatening).
Tip: keep in mind that half-a-percent of women are color blind, but as much as 8-9% of males have some degree of color blindness. Resorting to bold, italic, and exceptional use of uppercase will alleviate that potential problem!
Internet acronyms "BRB" (Be Right Back) or "IM[H]O" (In My [Humble] Opinion") are a great time saver, but they are not widely known to people who have not spent much time in chat rooms, online forums, etc. The other person's age and tech-savviness will often help you determine whether one of these acronyms is OK to use in your emails or not.
In doubt, be courteous and expand the acronym the first time you use it in an email as we did in this first line; if the colleague with whom you are emailing wasn't familiar with that acronym, you can use it plainly (unexpanded) next time, and assume that he/she will understand. But be wary of casual language in emails to superiors, since a casual writing style can be construed as a lack of respect.
More casual acronyms like "LOL" (let alone more vulgar ones along these lines!) should only be used when there is a certain level of familiarity with the email correspondent and/or a hierarchical closeness: never casually email the president of the company the same way you would a beer buddy, if you like the idea of a future with this employer.
"ASAP" is useful to communicate urgency, but its overuse has made it as unnecessary as it is an unpleasant; avoid using it at all cost. Here's a rule of thumb: if you type it more than once a month per email recipient, you are overusing it.