A spam filter is the system an inbox provider uses to decide where each incoming email goes: the inbox, a tab such as Promotions, the spam folder, or nowhere. Modern filters weigh many signals together rather than following a few fixed rules.
What filters look at
- The sender: authentication, domain and IP reputation, domain age, sending volume and history.
- The recipients' behaviour: replies and opens help; spam reports and deleting unread hurt.
- The email: links and where they point, images, attachments, formatting, and wording typical of spam.
- The list: bounces and spam traps.
What matters most for cold email
Reputation and behaviour outweigh the words. A clean, warmed-up sender writing short, plain emails to the right people reaches the inbox; the same email from a new, unwarmed domain may not. Content still counts at the margin: check a draft with the free spam word checker.