Hi PImoose!
There should be no reason to look for FreePBX related solutions. UCx uses postfix as its mail server - you just need correctly configured postfix and Exchange.
With a properly configured network, domain and DNS, the default configuration of postfix on UCx should send messages via the Exchange server without any additional configuration steps required. When postfix is started, it performs an MX lookup on the DNS Server and requests the mail server IP address. If an IP address is returned by the DNS server, postfix would use this address to route mail.
If your Exchange server is not receiving email messages from UCx, check the following:
1. The host name used for your UCx system uses the same domain name as the Exchange server
2. The domain name can be resolved using DNS on your network
3. The Exchange server is configured to allow messages from UCx (depending the version of your Exchange server, you may need to set up a receive connector for the IP address or host name of the UCx system)
If you still have problems, the output in maillog (available via the Support - System Log Files page in your GUI) should provide more info.