Generally the presiding judge decides what form of evidence can be submitted.If an e-mail can be connected to the person without there being any doubt of someone else having sent it, then it is...
Yes, from what I have seen on court TV shows. There is a lot of info in the headers that can make them reliable evidence.
Some people are bored when they send emails and want to see how many people get it. That's my best educated guess.
Much too general a question to answer. So much depends upon the type of case, and the prevailing 'rules of evidence.'
In the UK if it is evidence to a crime, Oh yes.