It means they are Shouting.
People only typed in CAPS (LIKE THIS). Have you heard of Google images?
All email addresses are in small caps, as are all internet addresses. If you are typing internet or email addresses in large caps, you are in fact "doing it wrong".
You either: A. Typed it wrong. For example: Jonjaliston@gail.om B. You had caps on. For ex. JONJALISTON@GMAIL.COM C. They dont like you. or D. The servers was shut down at the time.
Caps lock
no
caps and underlining
It can mean they're screaming at you, they have no clue that cap lock and bold are on, or they don't know the netiquette that says it's impolite to use all caps. Or the keyboard could be broken.
Basically, it's anything typed IN block capitals, but in a smaller point size. For example, your regular point size in your word processor could be 12 point. Small caps could be something typed in all upper case but with a point size of 8 or 6.
Never use caps.
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.
Any word that is all caps in email signifies that the writer is yelling.
Common unspoken rules. For example: ALL CAPS IS CONSIDERED YELLING>