There are exceptions, e.g., I sent an email full of research with sources to a family member I've been emailing with for fifteen years and it went to spam, despite it being @gmail.com to @gmail.com. In retrospect, I was misusing email versus sending a document or a link to one with it in.
The times have changed. It's not unreasonable to expect how people use email to have changed, ergo people sending emails full of URLs are statistically more often than not spammers.
Is it a broad stroke? Yup. But I'm willing to bet that I'm a fringe case and it prevents a ton of spam.
You can even test it yourself without fancy tools by sending emails full of links and mentioning keywords like Viagra et al.
For other filters/ISPs, content can matter a bit more, but as a general rule for consumer mailbox providers, metrics are the primary thing that affects filter outcomes.
It should go without saying that confirmation emails of a user initiated booking is not spam, but let's just say it anyway
[Edit]: Forgot to mention that the IP is home IP, not cloud provider/hosting or something.
And as as sidee note: the problem lies with the users and not with the email sender. For instance, if I send an important email to someone about an item they ordered from my site, and they tell me to send it to their gmail account, I connect to gmail to send the email, but if Gmail rejects it, I have fulfilled my part of the deal. If an email is marked as spam or rejected, it's on the recipient's end, not the sender's. The sender did not flag the message as spam before it is sent, the recipient did it after receiving it.
So 100% certainty of it being spam? Was that the correct judgment, in your opinion?
Some things are just baffling. How can they manage to flag themselves as spam it’s beyond me.