I have an interesting case that I am struggling with and any help would be amazing.
I have a client that is migrating from another ESP to the one I work at. The client was provided with a Dedicated IP and a specific Sender Domain for this new account. As a new client, we started the Reputation Period for the new IP and Sender Domain and used only the most engaged data available (started with Clickers - and openers - in the last 30 days).
From the first send, BT Internet started blocking the emails with the error message "smtp;554 Message rejected on 2019/12/03 12:28:04 GMT, policy (18.104.22.168) ID (5D6F8A0B128C2301) - Your message looks like SPAM or has been reported as SPAM". I knew the data was good so I contacted the Postmaster and asked for more information. I also visited the page "bt-email---best-practices-for-postmasters-and-senders-of-email" as I have seen before that you were providing this indication in other posts and ticked all the boxes.
Since then, I have basically sent an email almost every day requesting help to the Postmaster, contacted BT via Chat and Phone and I still do not have a reply.
A scenario that I have considered is the fact that the client, because of the migration process, is sending from both us and the other provider with different IPs and different Sender Domains. Can it be that BT is finding the emails SPAM because of the different senders? By the way, both IPs and Sender Domains have Return Path Certification so there should be a reassurance that they are both legitim.