I'm not using self hosted email. My theory is that Google treats smaller mail providers as less trustworthy and that increases the odds of having messages flagged for phishing.

In my case, the Google Search Console explicitly listed the exact URL for a newly created shared album as the cause.

https://photos.example.com/albums/xxxxxxxx-xxxx-xxxx-xxxx-xx...

I wish I would have taken a screenshot. That URL is not going to be guessed randomly and the URL was only transmitted once to one person via e-mail. The sending was done via MXRoute and the recipient was using GMail (legacy Workspace).

The only possible way for Google to have gotten that URL to start the process would have been by scanning the recipient's e-mail. What I was trying to say is that the only way it makes sense to me is if Google via GMail categorized that email as phishing and that kicked off the process to add my domain to the block list.

So, if email categorization / filtering is being used as a heuristic for discovering URLs for the block list, it's possible Google's discriminating against domains that use smaller email hosts that Google doesn't trust as much as themselves, Microsoft, etc..

All around it sucks and Google shouldn't be allowed to use non-deterministic guesswork to put domains on a block list that has a significant negative impact. If they want to operate a clown show like that, they should at least be liable for the outcomes IMO.