How do spam filters work?

Spam filters respond to patterns associated with unwanted email. Here are the most common triggers.

Authentication failures. Missing or failing SPF, DKIM, or DMARC. These are table stakes; failure signals untrustworthiness.

Reputation problems. Sending from IPs or domains with poor history. Appearing on major blocklists. Sending patterns similar to known spammers.

Content red flags. Excessive use of spam-associated words ("free," "guarantee," "act now"). ALL CAPS. Heavy exclamation marks. Suspicious URLs or link shorteners.

Technical issues. Missing unsubscribe links. Invalid From addresses. Malformed HTML. Excessive image-to-text ratios.

Sending behavior. Sudden volume spikes. Inconsistent sending patterns. High bounce rates. High complaint rates.

List quality signals. Hitting spam traps. Sending to many invalid addresses. Low engagement rates across the list.

Recipient actions. Being marked as spam by recipients is the strongest signal. Mass ignoring or deleting also feeds negative filtering.

Modern filters are sophisticated. They don't just look for banned words; they analyze context, relationships, and patterns. Gaming filters by avoiding specific triggers rarely works when fundamental practices are poor.