Spam filters analyze emails using a broad range of criteria, including scoring mechanisms, fingerprinting, and machine learning algorithms. No two spam filters look alike, so email senders must adapt to different approaches. Common reasons why legitimate emails land in the spam folder include poor authentication, sending IP reputation issues, domain reputation problems, recipient complaints, outdated or unclean email lists, form abuse, missing reply-to addresses, open URLs, suspicious attachments, and content that triggers spam filters. To avoid these pitfalls, senders must implement proper email authentication, maintain a clean email list, protect forms with CAPTCHA or honeypot fields, set up working reply-to addresses, include plain-text versions of HTML emails, use secure link shorteners, remove suspicious domains from links and images, and ensure domain alignment in the message body. By avoiding these common mistakes, senders can increase their chances of getting their emails delivered to the inbox rather than the spam folder.