I have a small web/mail server that hosts a very small number of low traffic sites for a few clients. A couple of months ago there was an outage in Google ReCaptcha that allowed some spam messages to be passed by a web contact form. Maybe a dozen or so spam messages got through in the space of an hour before I was alerted to the problem and took the form offline until ReCaptcha services returned. Two months later, messages to Gmail are still being bounced.
550-5.7.1 Gmail has detected that this message is likely suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. For more information, go to 550 5.7.1 https://support.google.com/mail/answer/188131
This was a very small incident and was contained nearly immediately. Why is Google still blocking mail for low reputation. When I check Postmaster Tools, I have no issues. The server isn't on any blacklist and hasn't been for years.
I've implemented all best practices including but not limited to: SPF, DKIM, and DMARC.
After submitting a service ticket through Postmaster Tools the block was lifted. I cannot post a link to the submit form because there is no deep link to it and the URL changes frequently.