so.. in the Settings panel,
a.)  under Bindings... you do not have anyports allocated to IPV6?
b.) under Protocol settings, the IPV6 is marked disabled?

c.) Also, how is your system set IP addresses on the SENDING email?

Is your system set to use the user's I/P address on the outbound or yours?

We too, have Spectrum business as one of the 5 connections that we use. Am assuming that you static I/P addresses from them (or from any provider).

There are settings specific to domains, under domains, then EDIT.  

Could be that your provider is using IPv6 for internal routing, or that they've forced an IPv6 address on you. Recommend reaching out to your provider and ask that they update the IPv4 address RDNS values to reflect what you're looking for. If of course, the settings above have been checked. 

Solution for Comcast IPv6 rDNS

 

IPv6 address are configured on the tunnel adapter. See the complete network details below. This article details how to disable that tunnel. See: http://winteladmin.com/disable-tunnelisatap-adapter/