Spam is any email that is not wanted or requested by the recipient. While the majority of spam comes from an unknown sender, oftentimes legitimate email is categorized by spam either by the recipient or by the ISP delivering emails to customer inboxes. So why does this happen?
Recipients who label your email as spam do so by clicking the “This is Spam” button in their email client. This notifies the ISP that this is unwanted mail. Each ISP has different thresholds or limits for the amount of acceptable complaints a sender can have for each email deployment. If you go above that threshold, you are likely to get blocked in the future. Having a high complaint rate is the number one reason why your email delivery will get blocked or delivered to the spam folder.