Outlook/Hotmail is the only service that's blocking my emails. I've been with DigitalOcean for 10 years. Beforehand it was just a matter of filling a form and waiting 24 hours to get the IP whitelisted. A few years ago, Microsoft started refusing whitelisting IPs.

DigitalOcean on the other hand started blocking SMTP by default for new customers since June/2022 [1], and thus significantly reduced the amount of spam coming out of their network. That said, they're still not doing enough to stop spam from their network, and they're still a source of spam [2].

I can cryptographically prove the identity of the server (and thus its reputation), and there's no justified reason to block mails based only on the network's IP address, while ignoring all the other factors.

1. https://www.digitalocean.com/blog/smtp-restricted-by-default

2. https://www.uceprotect.net/en/l3charts.php