Check if email address exists (not if email addrss is valid)

There is no way to reliably tell if an email address is
valid or invalid.
Some SMTP servers will simply always return OK for RCPT TO
to thwart
spammers.  Some SMTP servers won't even allow RCPT TO from
non white-
listed IP addresses...  You can't even expect a bounce
message for invalid
email addresses, some servers don't do that as well.

These characteristics are on the increase because of spam.