Name server timeout on relay

I am having a problem with my Sendmail server that is used to relay
email for my web application. When the application opens an SMTP
connection and begins to relay an email to certain email addresses,
Sendmail tries to do a DNS lookup on the domain being relayed to. The
lookup times out but not before my poor web application crashes from
its own timeout. So I have two questions:

Why is Sendmail doing a DNS lookup on an email address when it is
RECEIVING the email to be relayed? It seems like it wouldn't do a
lookup until it was time to deliver the email.

How can I stop the DNS lookup when accepting an email to be relayed?
Ideally, it would just accept the email and THEN do the DNS lookup on
the domain to be delivered to.

Please note that my Sendmail server is actually responsible for
delivering the email from the web app, so I can't just disable DNS and
have it forward all the email to some other relay server.

Thanks!
Tom

p.s. When running the same configuration on OSX Leopard running
Sendmail, the Sendmail server apparently operates perfectly without
timing out on the lookup.


Modified sample log:
*snip*
Aug 26 10:53:25 tick sendmail[29984]: m7QEq4CR029984: --- 451
bad_domain.com: Name server timeout
Aug 26 10:53:25 tick sendmail[29984]: m7QEq4CR029984: --- 050
<user@bad_domain.com>... Transient parse error -- message queued for
future delivery
Aug 26 10:53:25 tick sendmail[29984]: m7QEq4CR029984: --- 451
bad_domain.com: Name server timeout
Aug 26 10:53:25 tick sendmail[29984]: m7QEq4CR029984: --- 451
bad_domain.com: Name server timeout
Aug 26 10:53:25 tick sendmail[29984]: m7QEq4CR029984: --- 250 2.1.0
<user@bad_domain.com>... Sender ok
*snip*