I don't think that asking for money to agree to a divorce is actually illegal. Blackmail is demanding something from someone and then gaining from this demand. The demand must be unwarranted and ‘with menaces’ – a threat from the blackmailer to do something for not agreeing to the demand. The threat doesn’t have to be something illegal and doesn’t even have to be true. An example would be demanding £200 from someone in order to refrain from telling the world of their past adultery, previous criminal record or adultery.
There is a defence if the demand is made on reasonable grounds and the threat is a proper way to make the demand. This exists to cover those chasing legal debts such as banks when they send letters threatening repossession of a home unless they get paid.
Extortion is similar to blackmail. It involves obtaining money, property or services from another through threats of physical harm: “pay up or else” is an example. Protection rackets are a form of extortion.
Neither of these covers the situation described. The guy doesn't have to agree to the divorce, and repehensible though his conduct undoubtedly is, he has not actually made any threat to cause harm, moral, physical or financial, unless money is paid to him.
E-mail is a form of documentary evidence and can be admitted as evidence in court in the same way as can other forms of documentary evidence. However, as with other forms of evidence, the reliability of e-mail evidence will be subject to scrutiny.
