Sometime early last week we started getting error: "421 4.4.0 Remote server response was not RFC conformant" on any emails going to a few domains. After looking into it more I ran MX Lookups on the domains with errors, they are all hosted by Barracuda and I can't get help from them because I am not a client.
The only thing I can think of is our reverse DNS does not match our MX record. Our MX record points to our IP address, but a revers DNS points to our IP#.static.sonic.net
When I go into our host records in member tools, I can't find how to change the PRT or reverse DNS to match our MX record.