[Exim] Question on verification

Can someone explain to me what address verification feature (or some
other feature) in Exim would cause an error like the following?

[root@webhost root]# telnet mail.sourceforge.net 25
Trying 66.35.250.206...
Connected to mail.sourceforge.net.
Escape character is '^]'.
220 mail.sourceforge.net sc8-sf-mx2.sourceforge.net ESMTP Exim 4.34 Tue,
20 Jul 2004 16:29:17 -0700
MAIL FROM: bob@???
250 OK
RCPT TO: ipw2100-devel@???
550-Verification failed for <bob@???>
550-Unrouteable address
550 Sender verify failed
QUIT
221 sc8-sf-mx2.sourceforge.net closing connection
Connection closed by foreign host.


This system (linux.jf.intel.com) does NOT have an A record in DNS.

But it does have a valid MX record in DNS.

So why would Exim block this? This is a valid configuration to have and
as far as I know does not violate any RFCs.

Thanks,
John