I have been checking our site's email messages at http://www.mail-tester.com today to make sure we're not sending messages considered "spammy." All was well with several of our messages. Great!
But then I had to send an email from my GMail account to mail-tester.com. The message and subject of the email are generated from our site, and I wanted to make sure it's content wasn't "spammy," either.
The results (see image) surprised me, and now make me wonder: Has Gmail been blocking their own mail sent to my GMail account simply because their own sending IP addresses get blacklisted?
I've occasionally seen emails blocked by our SendGrid service that should get forwarded to my GMail account with a "550 5.7.1" reason. Now I have to wonder if they're blocked because they were sent through blacklisted GMail IP addresses. Am I totally off-base here?
Since there's been no response here or on Google's product forums, I'm now offering a 50 point bounty. It's as if I either asked the Wrong Question (as in "don't look behind the Google curtain!"), or it's wholly unknowable. At least it earned me that coveted Tumbleweed badge I've been hoping for! :/