Some E-mails are being rejected, do I need a DNS record for my default gateway?

Some E-mails are being rejected, do I need a DNS record for my default
gateway?

I have been seeing more of these types of bounceback messages (IP address
changed for example):
mail.test.com #550-Your message was rejected by this user and was not
delivered. 550-Reason: Your IP address [1.1.1.1] appears not to be an
email server. 550-Protection provided by: MagicMail version 1.3.5
(http://magicmail.linuxmagic.com) 550-For more information, please visit
the URL:
550-http://www.linuxmagic.com/best_practices/check_dynamic_reverse_dns.html
550-or contact your ISP or mail server operator. 550
ae9e5e74-0ad7-11e3-b831-0050569008a5 ##
We have all of the DNS records set up for our e-mail servers and smtp
outbound relay. Our MX record actually points back to our Barracuda
Spam/Virus Filter in front of our Exchange server. The IP address that the
message is referring to is our gateway address from our ISP.
What do I have to set up so that these messages are delivered? Do I set up
a fake MX record pointing to the default gateway? Add another A & PTR
record pointing to the default gateway? Is there something in our internal
configuration that could be incorrect? No records are set up for the
gateway address now, just the assigned IP addresses for each of the
outbound services. All servers are Windows based. Primary e-mail server is
Exchange 2007.