An email can be marked as spam for various reasons. Let's dive deep and learn more about them:

Subject line: Major ESPs like Yahoo, Gmail, and AOL have spam filters built into their service. These filters look for certain phrases in the subject line to determine which emails are spam and which are genuine. Some of these phrases include "weight loss," "FREE," and "earn from home." If an email's subject line has these phrases, it may be classified as spam.

Sender domain: Internet Service Providers (ISPs) use mechanisms similar to the ESP's spam filters to validate the sender's domain. The only difference is that the mechanisms used by ISPs are more powerful. When they find a sender domain that has been constantly sending spam, they will blacklist the sender domain. All the emails sent from that sender domain will be blocked. If your sender domain is blacklisted, you must contact the respective ISP and take necessary steps to whitelist your sender domain.

Content: Ensure that you offer relevant content to your contacts. While crafting an email, maintain the optimal ratio of 60% text and 40% images. Ensure that you don't use shortened URLs or URLs with multiple redirects in your email content.