We are seeing this for every email sent to a Shaw account:
"delivery temporarily suspended: host http://idcmail-mx1so.cg.shawcable.net [188.8.131.52] refused to talk to me:
554 http://mi06.dcs.int.inet cmsmtp Connection refused - nnn.nnn.nnn.nnn"
where nnn.nnn.nnn.nnn is a static IP on our mailserver.
We have no trouble with other mail servers, including Google, Outlook, Telus, Rogers, AOL, Yahoo etc.
We use valid DKIM and SPF and are not on any blacklists that we can find.
We slowed down the send rate to less than 500 messages per hour, and we only have ~ 2000 Shaw subscribers.