You can fix this under System Administration --> Return addresses.One of the options is the dmarc feedback address. Found it seconds after posting the first message :)

We have this problem as well. I see lines like this from the ironport log:Delayed: DCID XXXXXX MID YYYYYY to RID 0 - 4.1.0 - Unknown address error ('450', ['4.1.8 <MAILER-DAEMON@IronPort.OURDOMAIN.com>: Sender address rejected: Domain not found']) []...