Sounds like a job for your DNS provider, not ClearOS...Unless you're referring to the ability to put ClearOS in as your gateway, and route SMTP traffic back to your existing mailserver residing on the LAN segment of the ClearOS network...in that case, you have two options. First, simply create a port forward rule for SMTP back to the LAN IP of your mail server. That is done via "Network -- Port Forwarding". You need the port forwarding app installed, of course.
Alternatively, you can use ClearOS as a mail gateway filter/passthrough. In that case, you don't add the port forward rule..instead you open SMTP up on the Incoming Firewall (app installed by default). You also install the "SMTP" app from Marketplace in addition to any other sub-components (spam, malware, greylisting etc.). In your SMTP configuration, you'll set the mail forwarding entry to your existing mail server. Additional info on this configuration can be found here.