If it comes in to your server from AppRiver then you should be able to add an "IP Bypass" which will skip over AppRiver's IP and get the next part of the "Received By:" chain.
You can add an IP Bypass in the System Admin > Antispam section. This should hopefully resolve the DMARC issues.
The IP Bypass would be the IP of AppRiver's server that gives you the message.
Matt Petty
Senior Software Developer
SmarterTools Inc.
www.smartertools.com