alt Hacker NewsMy clients have been experiencing this forever; the logs SAY "temporarily rate limited due to IP reputation." but really the emails are never going to get delivered. I have to get MailChimp or Mailgun to rotate the IPs.
It looks like all it takes is one person to mark your email as spam, even by accident. Note that these are mailing lists which they signed up for in MailChimp case OR transactional emails in the Mailgun case.
It's only hotmail/outlook that we constantly have this issue with, Google etc. are all fine.
Often these "spam" reports by end users are just accidental clicks as well. Many of the abuse reports we get are like an email from someone's Mum and visibly legitimate. At other times there are users who use the Report Spam function as a kind of inbox management tool - a way of moving mail away so they don't have to see it because Trash or Delete or whatever is just further away from their pointer.