Sonicwall Firewall & Exchange 2010: DNS help

We have a Sonicwall TZ210 Network Security Appliance and are having a minor DNS issue related to email. When we send mail from our internal server the IP says [login to view URL] (one of our usable public IPs). However, all of the external connections to our server are made to [login to view URL] (the IP of the Sonicwall) which then forwards to the proper port internally. The problem is that some mail recipient servers are checking [login to view URL] against the FQDN of our domain ([login to view URL] and it resolves to [login to view URL]). They then reject our outgoing mail messages since the A record of [login to view URL] doesn't resolve to the 102 IP. We are trying to get all outgoing mail to show [login to view URL] on the header and need help to do this. We are using Exchange 2010 as our server.