Thaiseer, This is a great question! An email server identifies the spam by using spam filter software, which means it evaluates incoming emails on a number of criteria.
Do you know how to do anti-spam software/services detect to the spam? If no, I will tell you, They primarily utilize the different methods including the content-based message encoding and keyword filtering rules, email authentication rules. Actually, if anyone sent an email to your mail id, first it goes to the server that initially evaluates the IP and domain blacklist to make the status of email server authentication and then it will filter it (using the content rules). If an email is scored higher than calculated criteria then only the email is identified as spam mail and moved to the spam inbox of the receiver.
I hope certain words or the style of emails can send-up a red flag for spam filters, such as using lots of exclamation points, all caps, or all image emails.
I’m not good in this factor but I learned from the articles, if there anyone good in this part, they will explain you better than me 🙂