And this plays into the strengths of the big mail networks in detection. It's a bonus to them that every time they block a smaller host there is a good chance that sender will consider a move to office365 or Google Workspace for their mail.
As an aside, not sure if OP is related to them but updown.io is a nice service and I appreciate the simple PAYG pricing! For what it's worth their mails seem to get through successfully to me too.
Also for those facing mail delivery issues (or just practicing good email hygiene) - I recommend www.mail-tester.com - they give you an email address to send a mail to and carry out a heap of tests - including checking against SpamAssassin + blacklists, SPF/DNS/etc testing.