iCloud addresses blocking my work domain emails

Useful information. I have a domain-based email address set up by my hosting provider for my business domain. I have had this address for years. So it's "my" server, but I do not directly administer it, my provider does. As of January 28th, all mails from my work address sent to recipients with @icloud.com addresses are getting the 554 5.7.0 Blocked error. Messages to clients at other domains are working normally.

The problem being that mails sent from myaddress@fantastic-realities.com to recipients at anyaddress@icloud.com are blocked. Apple Support has today suggested to me that it is a server level problem at my email provider that they need to fix.

I have passed on the results (PASS) of the DMARC test to my provider.

Spamminess does not appear to be an issue, as the block seems to affect all mails from my domain address, even plain text, to @icloud.com users. But tested the first blocked message anyway.

Good stuff. Your email is almost perfect

Score : 8.8/10

And my IP Address did not appear against 24 of the most common IPv4 blocklists.

So seems to be a specific issue between my provider and iCloud's servers... arrrrrgggghhhhh....

Folow up - It was suggested to me that the address or my IP Address might have been used,  spoofed, or relayed for Spam and gotten my work email on a block list.