[Exim] Spam and RCPT verification...

Isn't exim setup by default to open an SMTP connection for the mail
server of the domain in the FROM: address of e-mails and verify that the
user exists?

ie:

From: cdaehn@???

pobox.com MX 10 mx-pa-1.pobox.com

So, exim makes a connection to the SMTP server and does this:

220 dolly1.pobox.com ESMTP Postfix
HELO foo
250 dolly1.pobox.com
MAIL From: exim-test@???
250 Ok
RCPT To: cdaehn@???
250 Ok
RCPT To: cdaebn@???
550 <cdaebn@???>: User unknown

so mail from cdaehn@??? would be accepted, but cdaebn@???
would bounce with a 550 error...

Is there an easy way to implement this? Surely it would kill alot of
the incoming spam by simply rejecting anything that doesn't verify... I
got the impression somehow that the particular feature in question was
the default behavior of exim, but it doesn't seem to be doing it... Or
am I just reading the instructions wrong again, as usual?