yes.
it is equivalent to yelling, or being angry...
The implication is one of impatience and shouting
Typing a word entirely in capital letters to emphasize a word when writing a message in an IM application or email. For example if you want to yell at someone over an email, then you write the whole message in capital letters and with exclamation marks.
No, it is virtually never appropriate.
A letter composed in all capitals appears to be shouting.
the person you're receiving the email from is shouting.
No, "e-mail" should not be capitalized; it is not a proper noun.
Misconstrue means to mistake the meaning. Maybe someone sends you an email that has capital letters in it, and so you think that they are yelling at you. Later you find out that the capital letters were just part of a really long acronym that is common at your new company. You misconstrued the email... meaning you misunderstood it. Here is the dictionary.com definition for more reading pleasure: http://dictionary.reference.com/browse/misconstrue
Spam letters are letters that advertise email.
It is rude, the equivalent of shouting. It is difficult to read, and does not allow for correct sentence structure. Something that people fail to realize about email is that your correspondents draw many conclusions about you from how your messages look. If they are not constructed as an educated person would, they will probably assume that you are uneducated. It's like walking around in public picking your nose, but most folks don't get it.
write it like you would a regular email
sometimes smileys, especially when sent thru outlook, become a capital J in the email.