There are more problems, you can't 'ping' an email address, just the email host, and some email hosts lie behind firewalls which may mean you can't ping the machine, even though it may accept SMTP traffic. Even attaching to and trying to verify the email address leads you into many of the problems discussed above, mainly the forging of addresses.

One of my old email addresses has been used for over a year as a forged from address. Because the old email address was supposed to forward to my new address, I was forced to begin using spambayes for outlook. I've been pretty happy with it, but need to upgrade. The version I'm currently using will very occasionally cause outlook to lockup (well, won't let me open any emails, and won't fully shutdown).

-Scott