Spam filtering is not based on one factor. Each mailbox provider uses its own algorithm, filtering rules, reputation data, security checks, engagement signals, and content analysis. A reliable deliverability workflow should review email content, authentication checks, reputation signals, technical metrics, and actual mailbox placement together.
Email content quality
Aggressive subject lines, misleading claims, excessive capitalization, broken formatting, too many promotional words, or low-quality content can increase your spam score. Spam words do not automatically send a message to spam, but they can contribute when combined with other risk signals.
Suspicious links and link reputation
Spam filters review the domains behind your links, redirects, tracking URLs, and landing pages. A risky link can hurt inbox placement even when the rest of the email looks clean.
Images and attachments
Image-heavy emails, missing alt text, large files, and risky attachments can all create deliverability problems. If you send PDFs, documents, or files, read our article on
email attachments and deliverability
before your next campaign.
Authentication problems
Missing, broken, or misaligned SPF, DKIM, and DMARC records can reduce trust. Authentication issues can make your messages look suspicious, especially when the From domain, sending server, and headers do not align.
Poor Sending Reputation
High complaint rates, low engagement, invalid recipients, spam traps, inconsistent volume, and sudden sending spikes can damage sender reputation. Reputation problems often appear first as provider-specific spam placement before they become a larger delivery issue.
Security and phishing signals
Mailbox providers also look for phishing patterns, spoofing, suspicious attachments, misleading links, and other threats. Strong authentication and consistent domain identity help reduce unwanted filtering.