Improper reverse lookup for Mail.

[Brett Simpson]

I currently have a mail server that uses 207.156.7.30
(mail.hillsboroughcounty.org) to receive email. When the mail server
sends out an email it uses 207.156.7.11 since it's the first bound
address on the server. This causes a problem with other receiving mail
servers using reverse lookups to help reduce spam. Since
mail.hillsboroughcounty.org is reversed to 207.156.7.30 and not
207.156.7.11 this causes a problem. What is the best way to handle this?
I can't eliminate 207.156.7.11 and I'd rather not force sendmail to only
use 207.156.7.30 since I'm looking to move this server behind a load
balance and failover system. Another mail admin suggested I use a CNAME
but I'm not sure how to do it or even if it will work.

Brett