I work for MailChannels. We have a few internal MTA hops while processing incoming email in our inbound filtering service. At each hop, we add a Received header as per the RFCs. Some of the internal MTAs are on private IP addresses. The 0.0.0.0:2500 Received header is the first hop into our edge SMTP proxy. While it's not super cool to have an IP of 0.0.0.0, we don't believe it's inconsistent with the RFC.
But please, if we're doing it wrong, we want to fix this and would be keen to know what you all think is the correct way to add headers.