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.