I think this is a big waste of time for the Mumbai police. If the terrorists can't send an e-mail with their threats, they will just send it by postal mail (just as they were doing before e-mail). Stopping them from sending anonymous e-mail won't stop the acts of terror. The Mumbai police should focus on investigating the actual attacks and preventing further attacks, rather than shooting the messenger.
Some people think that this can prevent them from coordinating their attacks, but I don't think so. Their attacks can be coordinated using various other techniques that may even be illegal - won't mention them, use your imagination.
Fundamentally, creating new rules will not stop terrorists - remember that there are already laws that prevent people from acquiring AK-47s & explosives. New rules will only inconvenience law abiding citizens - not terrorists.
Also, on another note - I don't like Times of India because they selectively prevent some comments from being displayed. I specifically mentioned this point in their comments and they have not published it, even after 2 days.