There isnt a configuration where
domain-1 sends from ip ending in 120
systemdomain sends from ip ending in 126?
No. Postfix uses just one of the domains of your server for sending, there is no connection between sender email address that you choose in your email client for a email to the IP address that postfix uses to send this email and thats absolutely ok as it does not matter for this problem.
its the sending IP address not matching the domain im emailing from is the problem.
No, thats not the problem. The problem is that your reverse dns of the IP address(es) do not match with the server hostname. The error is clear that you posted:
OK - xxx.xxx.xxx.120 resolves to otherdomain.com
or in other words, the IP address xxx.xxx.xxx.120 has a reverse record which points to otherdomain.com instead of mail.(serverboxname).com.
A postfix mail system identifies itself to other server always with the hostname of the server (mail.(serverboxname).com), the domain that is used as sender address does not matter here.