[m_ram]

This is probably obvious to people who do email marketing, but I just tested whether gmail bounces bad addresses. They do bounce addresses that have never been registered and addresses that have been deleted. This means that even before this change, spammers could find out if an address is active.

[herge]

Bouncing badly typed addresses is more useful for users than stymieing spammers. However, a spammer doesn't need to know if an existing gmail address is being actively used or not.