Something similar happened to Josh Marshall, who wrote an article about it: https://talkingpointsmemo.com/edblog/a-serf-on-googles-farm:
> So let’s review. We are paying customers of Google. We were forwarding emails from the site’s main address to all staffers. But because we receive a lot of spam, the spam that we were forwarding to ourselves marked us as a major spammer and led to Google banning all our emails with no notice in advance or notification after the fact.
> You might imagine that once we got through to someone at Google and explained this ridiculous situation they’d fix it. Well, no. Once we got through to someone they explained what happened. They told us a few remedial actions we could take. Once we did that, over time the algorithm would cease to think we were spammers.
The article also has this really apt quote for understanding Google and many companies like it:
> Google is so big and its customers and products (people are products) are so distant from its concerns that we’ve gotten caught up in or whiplashed by rules or systems that simply don’t make any sense or are affirmatively absurd in how they affect us. One thing I’ve observed with Google over the years is that it is institutionally so used to its ‘customers’ actually being its products that when it gets into businesses where it actually has customers it really has little sense of how to deal with them.
>Many of you know that we have one company email address here at TPM. It’s the one linked at the top of the site. It’s the lifeblood of the whole operation.
I also know from personal experience they used to send newsletter/marketing/transactional email from the same address, which you should never do. They probably got a little spammy with some marketing or newsletter emails and got their domain blocked by Spamhaus or another intermediary, and like every other ESP google has automated systems to blanket ban when they see you pop up in a spamhaus list. Pretty simple.