It means that there is 666 messages in your email inbox . Some sites that provides email services place a number there to let people know of their amount of unread messages , thus making their service easier for others to use
No not at all. In fact if it was any other number you probably wouldn't have noticed it as well. When there are numbers we here a lot about in our lives (Like 666) they tend to stick out when we see them in every day life. We tend to forget they are in fact a number and that you need to pass it to get to 667 (If counting by ones).
It is just a number like any other. If you wish to take notice of the symbolism, check out numerology. Every number can hold significance if we desire them to. For instance, i have been seeing the numbers 7, 17, 27 everywhere. When ever i tell anyone they usually give me a different view of their significance. This is obviously because they call out differently to other people.