The solution to "my transactional email service does not deliver to one ISP" is "use a transactional email service". I have used transactional email services for low volume and highly variable sites and have had very few problems, none of them general blocks - there were warnings from providers and a single digit number of hard bounces when one site had a vulnerability to registration spam (owner thought the previous developers had a honeypot to stop it, turned out it not so).

Its very weird that low volumes are the problem. I have been self-hosting personal email for myself and a few family members (so very low volume) on an OVH VPS for years. I cannot deliver to Hotmail (MS hosted institutional email works, outlook.com works) but that is the only problem I have encountered myself. The heaviest sender in the family had emails rejected by one business.