So I had/have this problem with a client that uses HubSpot for CMS, Sales, and Marketing. I found (as others shared on this thread) that more than 1 of the shared IPs were on the blocklist. I did a few tests with MailChimp and found much better deliverability. They use HubSpot for everything so migrating off HubSpot was a big decision.
We ended up subscribing to a private IP and using MailChimp until it was warmed up. It still isn’t great, but it is MUCH better than the shared IP. We still supplement with MailChimp for contacts that are considered unengaged by HubSpot. We also target those contacts through LinkedIn targeted ads trying to get them to engage.
It is really frustrating and asking the customers to allowlist the IP one-by-one is a non-starter. Hope this helps.