How do I prevent bounce-backs while sending mail via local smtp server, but receiving mail through Google Apps Domain?

We recently migrated to Google Apps and are sending and receiving mail through Google now.  However, I have several users who often send out emails to distribution lists of 600 - 800 addresses.  Google limits are set at 500 so I configured these users to send out mail using our own smtp server so they could get around these restrictions.  However, it appears that two to three ISPs or domains are blocking emails because of the following reasons:


host mx2.hanmail.net[211.43.197.35] said: 454 5.7.1 DXNS3 201.218.3.198: Message refused. Your host name dosen't match with your IP address. If you need, please contact stamp@daumcorp.com. (in reply to end of DATA command)

OR...

host mxs.pchome.com.tw[211.20.188.150] said: 450 4.7.1 Client host rejected: cannot find your hostname, [201.218.3.198] (in reply to RCPT TO command)


I have already tried to contact daumcorp.com about how to fix this, but they haven't responded.  I'm sure the issue is related to how we are sending mail through our own mail server and then receiving mail from Google, but I just don't know enough about this stuff to fix it.  Thanks for your help.