Gmail filtering in a nutshell

Gmail’s approach to filtering; as described by one of the old timers. This person was dealing with network abuse back when I was still slinging DNA around as my job and just reading headers as a hobby.

Gmail uses a 10+ year old neural network that analyzes thousands of factors, related to email, IP, and web, integrated with all Google products, and with 99.9%+ accuracy for identifying certain types of messages, combined with an email-specific domain-based reputation system that combines IP reputation, content, read rates, reputation of other senders with similar content.

This excerpt was shared with a bunch of delivery experts and every one of them agreed. The Gmail filters are incredibly complex and they measure thousands of different things about email. Yes, sometimes you can remove a link or a URL and get mail to the inbox for a while. That doesn’t mean the block was against the URL, simply that changing the URL changed the score enough for the mail to go to the inbox.
This is part of what makes Gmail delivery issues so difficult to troubleshoot. There isn’t one thing, it’s all the things that contribute to where an email ends up. We, as senders and deliverability experts, don’t have access to the Gmail data. The poster goes on to say:

Trying to fix this using only inaccurate proxy data where there is no mediation pathway in a matter of weeks is complex.  We consume data from a multitude of sources, compile and analyze the data, determine which of the hundreds of factors we can influence should be adjusted, come up with the easiest plan to address the most influential factors, and explain that to the customer using the clearest language possible to individuals who are not educated on the definition of a complaint.

We do our best, with limited data and try and tell you how to fix things.
One of the biggest challenges with Gmail delivery is I am convinced they look at your profile of recipients. They can map someone who is collecting addresses through third parties, or buying lists based on the specific Gmail accounts targeted by a mailing. Gmail has publicly stated and has on their website that they don’t think co-reg or purchased lists are opt-in. They have the technology and ability to track that. I think it’s one reason senders trying to use email for acquisition have such a challenge getting into the inbox and Gmail. I think it’s a feature, not a bug for them.