Understanding what spam filters actually measure
+ +Modern spam filters do not just scan for obvious keywords. They evaluate a combination of signals — your sending domain reputation, your authentication records, your content patterns, your engagement history and your list quality. A message can fail filtering for any one of these reasons, and the same message may be delivered to some recipients while filtered for others, because inbox providers use per-recipient reputation data.
+ +This means there is rarely a single fix. Understanding which signal is causing the problem is the first step.
+ + + +Cause 1: Missing or misconfigured authentication
+ +If your sending domain lacks valid SPF, DKIM and DMARC records, major inbox providers have no reliable way to verify your identity. Gmail and Outlook both use authentication status as a primary filtering signal. A missing or broken DKIM signature is one of the most common causes of increased spam placement.
+ +How to diagnose: Send a test email to a mail-tester.com address and check the resulting score. Any authentication failures will be clearly flagged. Also check your DMARC aggregate reports for authentication failure rates.
+ +How to fix: Implement SPF, DKIM and DMARC correctly. See our DMARC, DKIM and SPF setup guide for a step-by-step walkthrough.
+ + + +Cause 2: New domain with no established reputation
+ +A domain registered recently has no sending history. Inbox providers have seen enough new domains used for spam campaigns that they treat all new domains with caution by default. Starting at high send volumes immediately will typically result in significant filtering.
+ +How to diagnose: Check the registration date of your sending domain. If it is under 3 months old and you are seeing filtering issues, reputation is almost certainly a factor.
+ +How to fix: Use a domain warming process — starting at modest volumes and increasing gradually over several weeks. The goal is to build a history of consistent, legitimate sending before reaching high volumes.
+ + + +Cause 3: Hard bounce rate too high
+ +Hard bounces — sends to addresses that do not exist — damage your domain reputation with every campaign. Inbox providers treat a high bounce rate as a signal that your list was not collected legitimately or that it has not been maintained. Many teams treat 2% hard bounces as a warning benchmark — if you exceed it in a campaign, it is worth pausing to investigate before sending again. The exact threshold that triggers filtering varies by inbox provider and sending volume.
++How to diagnose: Review your bounce report for the last 5 campaigns. Calculate the percentage of sends that hard bounced.
+ +How to fix: Validate email addresses at the point of collection. Remove addresses that hard bounced in a previous campaign immediately. Do not mail contacts who have not engaged in over 12 months without first running a re-engagement campaign.
+ + + +Cause 4: Spam complaint rate above acceptable levels
+ +When a recipient marks your message as spam, that signal is sent directly to the inbox provider. Consistently high complaint rates lead to increased filtering across your domain. Complaint rates around 0.1% are commonly treated as an early warning sign worth investigating. Google Postmaster Tools provides actual complaint data for Gmail recipients — it is the most accurate source available to senders.
+ +How to diagnose: Check Google Postmaster Tools for your domain. If you do not have it set up, do that first — it is the most accurate source of Gmail complaint data available to senders.
+ +How to fix: High complaint rates usually indicate one or more of: content that does not match subscriber expectations, sending too frequently, list quality issues or contacts who never genuinely opted in. Address the underlying cause rather than just watching the metric.
+ + + +Cause 5: Content and structural spam patterns
+ +Certain patterns in email content trigger spam filters regardless of sender reputation. These include but are not limited to: excessive use of capitals in subject lines, too many hyperlinks relative to body text, image-heavy emails with minimal text, URL redirects through multiple domains, and certain phrase patterns historically associated with spam.
+ +How to diagnose: Run your email content through a pre-send spam checker before sending. NexusProMail scores emails 0-100 and flags specific issues. Mail-tester.com will also score a real send.
+ +How to fix: Address flagged issues before sending. This usually means adjusting content balance, reducing link density or rewording flagged phrases. The goal is a message that reads as a genuine communication, not a broadcast.
+ + + +Cause 6: Low engagement signals from recipients
+ +Inbox providers track how recipients interact with your email — opens, clicks, replies, and what percentage delete without opening or move to spam. A large proportion of your recipients consistently ignoring your messages sends a negative signal. Some providers factor this into inbox placement decisions not just for the ignoring recipients, but across the sender domain.
+ +How to diagnose: Segment your list by last engagement date. What percentage of your list last opened or clicked an email more than 6 months ago? More than 12 months?
+ +How to fix: Stop sending to disengaged segments. Run a re-engagement campaign to a subset — ask them directly whether they want to continue receiving email. Those who do not respond within a reasonable window should be suppressed. A smaller, engaged list consistently outperforms a large, unengaged one in deliverability terms.
+ + + +Cause 7: Sudden volume spikes
+ +Inbox providers notice unusual patterns in sending behaviour. A domain that normally sends 500 emails per day and suddenly sends 50,000 in a single afternoon looks unusual — even if the content and authentication are legitimate. This is particularly relevant for accounts that go quiet for weeks and then resume at high volumes.
+ +How to fix: Send at consistent volumes and increase gradually. If you need to run a large campaign after a period of low activity, ramp the volume up over several days rather than sending all at once.
+ + + +Diagnostic checklist
+ +- + +
- Check authentication: run a mail-tester.com test and review all sections + +
- Check domain reputation: set up and review Google Postmaster Tools + +
- Review bounce rate for last 5 campaigns + +
- Check complaint rate via your platform or Postmaster Tools + +
- Run a content spam check before next send + +
- Segment list by engagement date — identify inactive contacts + +
- Review DMARC reports for authentication failures + +
- Check whether any recent campaigns had unusual volume patterns + +
What does not fix the problem
+ +A few approaches that do not address the underlying issue: asking recipients to add you to their whitelist (this helps individual recipients but does not repair domain reputation), switching to a new domain without fixing list quality (the new domain will encounter the same problems), and reducing send frequency alone (this slows the damage but does not repair reputation).
+ +Consistent inbox placement comes from building and maintaining genuine sending reputation — authenticated domains, clean and engaged lists, relevant content and infrastructure that automatically handles bounces and complaints.