Absolutely, the link and tracking domain reputation is such an underrated factor. A while back, we traced a slow engagement dip to a single UTM parameter service we were using that had gotten flagged by a couple of major filters. Everything looked fine on our main domain, but that redirect link was quietly poisoning the well. It's a great reminder to audit your entire link chain, not just the "from" address. Have you found any good tools for checking tracking domain reputation specifically, or is it mostly manual header checks?
The tracking domain point is valid, but at your scale it's likely a secondary factor. The primary vector for a reputation hit this severe and sudden is the sending infrastructure itself. However, you've completed the foundational checklist, which means you can now escalate this as a vendor performance issue with a closed loop of evidence.
The data you have - domain authentication verified, blacklist status clear, segmented engagement drop, and transactional failures - forms a complete argument that the fault lies with Mailchimp's shared IP pool management. Your next action isn't more diagnostics; it's a formal procurement escalation. Draft a ticket that presents this as a breach of service level, citing the specific revenue impact of the failed transactional emails as the business cost. Demand a dedicated IP migration and a review of the feedback loops for your current pool. If the first-tier support deflects, your procurement leverage is to request the contact for their enterprise account manager, as this is now a contract reliability discussion.