I also used to work on email at scale (XXX million per day), and even then I encouraged everyone to click spam[1] for any email they did not expect to receive and do not want.
My email address and phone notifications are a direct link to me and are sacred in terms of getting my attention. Yet many companies confuse getting my email address with permission to mail me anything at any frequency. They also believe that "unsubscribe" should be able to redirect me to a website with a bunch of confusing checkboxes. Neither is acceptable.
So, when I click "unsubscribe", that should immediately unsubscribe me forever with no extra efforts, but so many emailers don't do this that I just gave up and started reporting spam.
> emoji in the headline was the campaign that caused 9% of people to mark spam and 20% to unsubscribe, but it was enough to have us blackholed for 2.5 months
A guess with no info, but it sounds like a case of not removing unengaged recipients, then suddenly getting their attention with a different looking subject line or an email that came at an unexpected time.
If people receive bulk for a long time and they never engage, remove them from the list or pause sending to them to avoid this effect.
[1] - For those who don't know much about email, two things should happen when you click the spam button in Gmail and select "Unsubscribe and report spam" (it's been a while, correct me if this changed):
1) List-Unsubscribe should be triggered and this should unsub you from the list. In 2023, I personally expect this to be immediate and receive no more email but I give senders a window of a day or so.
2) The FBL should give the sender a signal that this specific campaign was considered spam.