HTML allows the writer to emphasis important points in an email message. Although misspelled words are one common feature of email messages, you tend to be more forgiving of the occasional misspelled word and less forgiving of emails filled with grammatical abuses. Too many capitalizations, italics and word underlinings make your reader unable to decipher what is and what is not important in the email. All these errors reflect badly on you, the writer.
When you use all caps in an email, it is tantamount to screaming or shouting at your reader. Even in a short message, capitalizing every word in an email demonstrates a laziness on your part or a lack of respect for the reader. Your email should be written the same way you would type a business correspondence, with both upper and lower case words employed and correct punctuation and spelling.
When you use too much bold type, it makes your email difficult on the reader's eyes and unpleasant to read. Boldface type used alone in an email is self serving and shows a level of superiority of the writer over the reader in terms of her grasp on the subject matter. When you boldface every word, you are telling the reader that he needs help understanding your message. Do not insult the reader of your email by using too much boldface type.
Use bold sparingly in an email with no more than one bold sentence in a paragraph or just a few words in a sentence.
Too much underlining or too many uses of italics indicates that you do not have a clear point of view in the email. Truly bad users of HTML add insult to injury by underlining words or sentences that have already seen too much capitalization, bolding or italicizing. When you use too much underlining, you are telling your reader that you think she is incapable of comprehending your message.
While all these grammatical elements have their place from time to time in an email and do much to help the reader bring emotion and communicate a passionate discourse in their email, using them improperly is bad email etiquette and poor communication.