When legitimate email ends up in spam, it is frustrating for both senders and recipients. Understanding why spam filters make mistakes helps you improve deliverability and recognize when spam filtering is working as intended.
Sender reputation
The most important factor in spam filtering is sender reputation. If the sending domain or IP address has a history of sending spam, future messages from that source are treated with suspicion. Reputation is built over time through user complaints, spam trap data, and delivery statistics.
Disposable email domains are often rated poorly by reputation systems because they are associated with low-quality signups and abuse. This is why email sent from disposable addresses is more likely to be marked as spam.
Content analysis
Spam filters analyze email content for patterns associated with spam:
- Subject line patterns (all caps, excessive punctuation, promotional language)
- Body content (excessive links, keyword stuffing, formatting issues)
- Image-to-text ratio (emails that are mostly images are suspicious)
- Trigger words (free, win, guarantee, urgent)
- HTML quality> (broken HTML, missing alt text, malformed structure)
Engagement metrics
p>Spam filters also consider how recipients interact with email:- Open rates — very low open rates suggest spam
- Click rates — no clicks on links may indicate irrelevance
- Spam complaints — users marking messages as spam directly hurts reputation
- Delete-without-reading rapid deletion without engagement signals spam
Authentication failures
Emails that fail SPF, DKIM, or DMARC authentication are treated as suspicious because they may be forged or tampered with. Proper authentication is increasingly expected for legitimate senders.
Recipient behavior
Spam filters learn from individual recipient behavior. If a user frequently marks similar messages as spam, the filter learns to categorize those messages as spam for that user specifically. This is why what lands in spam varies by person.
Improving deliverability
For legitimate senders, avoiding spam filtering requires:
- Implementing SPF, DKIM, and DMARC authentication
- Maintaining good sender reputation by honoring unsubscribe requests
- Sending relevant, solicited content to engaged recipients
- Following email best practices for HTML, images, and formatting
- Monitoring spam complaints and blacklist status
Temporary email and spam
Temporary email inboxes are designed to block spam features that legitimate senders expect. Remote images are blocked to prevent tracking, HTML is sanitised to prevent malicious code, and messages are deleted automatically. This is a feature, not a bug — it protects the user, not the sender.
When you use temporary email to receive messages, you may see different behavior than in a regular inbox — tracking pixels will not fire, images may appear as placeholders, and rich HTML may be simplified. This is intentional and protective.