Getting your emails delivered to the inbox is harder than it used to be. Modern spam filters combine dozens of signals — from DNS authentication records to engagement rates — to decide where a message goes. Understanding each signal is the first step to fixing deliverability problems.
1. Missing SPF Record
SPF (Sender Policy Framework) is a DNS TXT record that tells receiving servers which IP addresses are authorised to send email for your domain. If your domain has no SPF record, receiving servers have no way to verify you are a legitimate sender — and many will treat the message with suspicion or route it to spam.
Check whether your SPF record exists using the ShowDNS TXT Lookup tool. A valid SPF record looks like:
example.com. IN TXT "v=spf1 include:_spf.google.com ~all"Fix: Publish an SPF TXT record at your root domain. See our SPF setup guide for step-by-step instructions.
2. Failed DKIM Signature
DKIM (DomainKeys Identified Mail) adds a cryptographic signature to outgoing messages. The receiving server retrieves your public key from DNS and uses it to verify the signature. A failed DKIM check — caused by a missing key, a broken key record, or mail being modified in transit — is a strong spam signal.
Common DKIM failure causes:
- The DKIM TXT record was never published in DNS.
- The record was published under the wrong selector or subdomain.
- A mailing list or forwarding service modified the message body, breaking the signature.
- The private key used to sign was rotated but the DNS record was not updated.
Verify your DKIM record is published. DKIM records follow the format selector._domainkey.yourdomain.com — for example, google._domainkey.example.com. Query it with:
dig TXT google._domainkey.example.com +short3. No DMARC Policy
DMARC is the policy that ties SPF and DKIM together and signals to receivers how to handle failures. Without a DMARC record, you are missing the layer that prevents spoofed email from reaching your recipients' inboxes. Some receivers — particularly large providers like Gmail and Yahoo — now require a DMARC record as a condition of inbox delivery for bulk senders.
Fix: Publish a DMARC TXT record at _dmarc.yourdomain.com. Start with p=none while monitoring, then escalate to quarantine and reject. See our DMARC configuration guide.
Since February 2024, Gmail and Yahoo require bulk senders (over 5,000 messages per day) to have a valid DMARC record with at least p=none. Senders without a DMARC record risk having messages rejected or routed to spam.
4. Poor Sender Reputation
Every sending IP address and domain accumulates a reputation score based on past sending behaviour. A low reputation score is one of the most common and hardest-to-fix spam causes. Factors that damage reputation include:
- High bounce rates (sending to invalid addresses).
- High spam complaint rates (recipients clicking “Mark as spam”).
- Sudden spikes in sending volume.
- Sending from a brand-new IP with no history (also called a “cold” IP).
- Previously sending spam or being compromised.
Fix: Clean your mailing list regularly to remove invalid addresses. Keep complaint rates below 0.1%. Warm up new sending IPs gradually by starting with small volumes and increasing over several weeks.
5. Blacklisted IP Address or Domain
DNS-based Blocklists (DNSBLs) are real-time databases of IP addresses and domains known to send spam. If your sending IP or domain appears on a major blocklist — such as Spamhaus, Barracuda, or SORBS — many receivers will automatically route your mail to spam or reject it entirely.
Diagnose: Check your sending IP against the major blocklists using MXToolbox Blacklist Check or a similar tool. Enter your outbound mail server's IP address.
Fix: If listed, identify the cause (compromised account, outbound spam, open relay) and remediate it. Then submit a delisting request to each blocklist operator. Delisting can take 24–72 hours.
6. Spam Trigger Words and Content
Content-based spam filters analyse the subject line, body, and HTML structure of your message. Common content triggers include:
- Subject lines with excessive punctuation or ALL CAPS.
- Words like “FREE”, “GUARANTEED”, “CLICK HERE”, “ACT NOW”, or “CONGRATULATIONS”.
- Excessive use of exclamation marks.
- Messages that are primarily images with little text (hiding spam content in images).
- Mismatched link text (the visible text says one URL but the link goes to another).
- HTML that renders differently in different clients due to poor coding.
- Invisible text (white text on white background) used to stuff keywords.
Fix: Write natural subject lines. Maintain a healthy text-to-image ratio (aim for at least 60% text). Ensure all links point where they say they do. Validate HTML with a tool like Litmus or Email on Acid.
7. Missing or Broken Unsubscribe Link
For marketing and bulk email, a one-click unsubscribe mechanism is legally required in many jurisdictions (CAN-SPAM, GDPR, CASL) and is a deliverability best practice. Gmail and Yahoo also require bulk senders to support one-click unsubscribe via the List-Unsubscribe and List-Unsubscribe-Post email headers.
When recipients cannot find an easy way to unsubscribe, they click “Mark as spam” instead — which directly damages your sender reputation.
Gmail and Yahoo require bulk senders to honour unsubscribe requests within two days. Your marketing platform should support this automatically, but verify that the List-Unsubscribe-Post header is present in your outgoing headers.
8. Low Engagement Rates
Gmail, Outlook, and other providers track whether recipients open, reply to, or click links in your messages. If a large proportion of your recipients consistently ignore your emails, the provider treats that as a signal that the messages are unwanted — and routes future messages to spam.
Fix: Segment your list and send only to engaged subscribers. Remove recipients who have not opened an email in 6–12 months. Focus on sending relevant, personalised content that recipients actually want to receive.
9. HTML and Technical Issues
Poorly structured HTML is a red flag for spam filters. Issues that can trigger filters include:
- No plain-text alternative (multipart/alternative is required for good deliverability).
- Broken or malformed HTML tags.
- Inline CSS that conflicts with email client rendering.
- External resources (images, scripts) loaded from suspicious domains.
- Attachments with unusual file types (executables, encrypted archives).
How to Diagnose Your Deliverability Problems
Use these tools to identify which issues affect your domain:
- ShowDNS TXT Lookup — verify SPF and DMARC records are published correctly.
- ShowDNS MX Lookup — confirm your MX records are configured and pointing to the right mail servers.
- Mail-tester.com — send a test email and receive a spam score with detailed explanations of every issue found.
- Google Postmaster Tools — monitor your domain's reputation, spam rate, and DMARC pass rate directly within Gmail.
- MXToolbox Blacklist Check — check whether your sending IP is on any major blocklist.
# Check SPF record
dig TXT example.com +short
# Check DMARC record
dig TXT _dmarc.example.com +short
# Check DKIM record (replace 'google' with your selector)
dig TXT google._domainkey.example.com +shortFrequently Asked Questions
My SPF record is correct but emails still go to spam. Why?
SPF is only one factor. Check DKIM (is it signed and passing?), DMARC (is the policy published?), sender reputation (is the IP blacklisted?), and content (are there spam triggers?). All of these factors contribute to the final spam verdict.
How do I check if my IP is blacklisted?
Use MXToolbox's free Blacklist Check tool at mxtoolbox.com/blacklists.aspx. Enter your outbound IP address and it will check it against over 100 DNSBLs simultaneously. Note that your outbound IP may differ from your website's IP.
Does the From name or address affect spam filtering?
Yes. Using a From address that does not match the domain in your SPF/DKIM configuration breaks DMARC alignment and triggers spam filters. Also avoid no-reply addresses where possible — they signal low engagement to filters.
How quickly can I improve my sender reputation?
Reputation recovery takes weeks to months. Focus on removing bad addresses, reducing complaints, and sending only to engaged subscribers. Google Postmaster Tools gives you direct visibility into your reputation score with Gmail.
Can I test my email before sending?
Yes. Mail-tester.com generates a unique test address. Send your campaign to that address and get a deliverability score (out of 10) along with specific issues to fix. Aim for a score of 9 or above before sending to your real list.