Why your emails go to spam - and how to fix it.

1. How spam filters actually work

Modern spam filters are not simple keyword detectors. They are machine learning systems trained on billions of emails that evaluate hundreds of signals simultaneously to decide whether your email belongs in the inbox, spam folder, or should be blocked entirely.

Gmail, Outlook, Yahoo and Apple Mail each run their own filtering systems, but they share a common architecture. When your email arrives at their server, it is evaluated across three categories: authentication signals, reputation signals, and content signals.

Authentication signals - can we trust who sent this?

Before a spam filter even looks at your content, it verifies that you are who you claim to be. This is done through three DNS-based protocols: SPF, DKIM and DMARC. If any of these fail or are missing, your email immediately receives a negative score that can be difficult to overcome with good content alone.

Reputation signals - has this sender been problematic before?

Your sending domain and IP address have a reputation score maintained by each inbox provider. This score reflects your historical behaviour - bounce rates, spam complaint rates, sending volume patterns, and blacklist status. A new domain starts with a neutral reputation. How you behave in the first 30-60 days of sending largely determines your long-term reputation.

Content signals - does this look like spam?

Content analysis looks at subject lines, body text, HTML structure, image-to-text ratio, link destinations, and more. Modern filters are sophisticated enough to understand intent rather than just matching keywords - a medical professional sending information about medication will not be penalised the same way as a spammer using the same words.

2. Sender reputation - the root cause of most deliverability problems

Sender reputation is the single most important factor in email deliverability. Every other problem - bad content, authentication failures, blacklisting - ultimately damages your reputation. And once reputation drops, everything suffers.

How reputation is measured

Google's Postmaster Tools (free) shows your domain reputation as one of four levels: Bad, Low, Medium and High. Only High reputation results in reliable inbox placement. Microsoft's SNDS (Smart Network Data Services) provides similar data for Outlook and Hotmail delivery.

What damages reputation fastest

• High bounce rates - hard bounces above 2% signal that you're sending to addresses without permission or without list maintenance
• Spam complaints - when recipients click "Report spam", it directly impacts your reputation. Google's threshold is 0.1%. Above 0.3% triggers severe filtering.
• Spam trap hits - sending to spam trap addresses (specially configured addresses used to catch bad senders) is one of the fastest ways to get blacklisted
• Sudden volume spikes - sending 50,000 emails from a domain that normally sends 500 looks suspicious to spam filters

3. Missing authentication - SPF, DKIM and DMARC explained

Email authentication is not optional in 2026. Google and Yahoo made SPF, DKIM and DMARC mandatory for bulk senders in February 2024. Without them, your emails will either be filtered or rejected entirely.

SPF - Sender Policy Framework

SPF is a DNS record that lists all the servers authorised to send email on behalf of your domain. When an inbox provider receives an email claiming to be from yourcompany.com, it checks your SPF record to verify the sending server is on your approved list.

A typical SPF record looks like:

v=spf1 include:_spf.google.com include:sendgrid.net ~all

The ~all at the end means "softfail" - emails from unlisted servers are accepted but marked as suspicious. Use -all for hard fail if you want stricter enforcement.

DKIM - DomainKeys Identified Mail

DKIM adds a cryptographic signature to every outgoing email. The receiving server uses your public key (published in DNS) to verify the signature. This proves the email genuinely came from your domain and was not altered in transit.

Most email service providers (Mailchimp, Brevo, SendGrid, Klaviyo) generate DKIM keys for you and give you a DNS record to add. Check your ESP's settings for "DKIM" or "domain authentication".

DMARC - Domain-based Message Authentication

DMARC tells inbox providers what to do when an email fails SPF or DKIM. It has three policy levels:

Start with p=none and add a rua tag to receive daily reports: v=DMARC1; p=none; rua=mailto:[email protected]

4. Content triggers that flag spam filters

While authentication and reputation are the biggest factors, content still matters. Modern spam filters analyse the full context of your email, not just individual words.

Subject line mistakes

• ALL CAPS subject lines - shouting is a classic spam pattern
• Excessive punctuation - "Buy NOW!!!" or "Free????" both trigger filters
• Classic spam phrases - "Make money fast", "100% free", "Act now", "Limited time offer" still carry negative weight
• Misleading subjects - "Re:" or "Fwd:" when it's not a reply signals deception

Body content issues

• Image-only emails - emails with no text content and a single large image look like spam. Maintain a 60% text to 40% image ratio.
• Too many links - more than 3-5 links in a short email raises flags
• URL shorteners - bit.ly and similar services hide the destination URL, which spam filters distrust
• Mismatched links - link text says one domain but the href goes to another
• Invisible text - white text on white background, used by spammers to stuff keywords

5. List quality problems

This is where most deliverability problems actually originate. A clean list is the foundation of inbox placement. No amount of good content or correct authentication will save you if you're sending to thousands of invalid, abandoned or spam-trap addresses.

How lists go bad over time

Email list decay is real and constant. Industry research consistently shows that approximately 22% of email addresses become invalid or unusable every year. The reasons include people changing jobs and losing their work email, people abandoning personal accounts, companies shutting down, and people deliberately entering fake addresses to access gated content.

A list you collected 18 months ago without any cleaning is likely 25-30% invalid. Every campaign you send to that uncleaned list is actively damaging your sender reputation.

Signs your list has quality problems

• Bounce rate above 2% on any campaign
• Open rate declining month over month without obvious cause
• Spam complaint rate creeping above 0.05%
• Large numbers of addresses from known disposable providers
• Many role-based addresses (info@, noreply@, admin@) that were never real people

6. Email blacklists - how to check and get off them

An email blacklist is a database of IP addresses or domains that have been identified as sources of spam. Inbox providers and spam filters check these lists as part of their filtering decisions. Being on a major blacklist can cause immediate and severe deliverability problems.

The most important blacklists

• Spamhaus - the most influential. Being on the Spamhaus Block List (SBL) causes rejection at most major inbox providers. Check at spamhaus.org/lookup.
• Barracuda - widely used by corporate email gateways. Check at barracudacentral.org/lookups.
• SURBL - domain-based list used for URL checking within email content
• MXToolbox - free tool that checks your domain and IP against 100+ blacklists simultaneously

How to get delisted

Each blacklist has its own delisting process. The general approach is: first fix the underlying cause (stop the bounces, remove the spam traps, correct the authentication), then submit a delisting request with evidence of what you've fixed. Spamhaus typically responds within 24-48 hours for legitimate requests. Barracuda has a self-service delisting tool.

7. Low engagement signals

Gmail in particular uses engagement signals as a strong inbox placement factor. If your subscribers consistently ignore your emails - never opening, never clicking - Gmail interprets this as evidence that the emails are unwanted.

What counts as engagement

• Opens (though increasingly unreliable due to Apple Mail Privacy Protection)
• Clicks within the email
• Replies to the email
• Moving an email out of the spam folder
• Marking an email as "not spam"
• Adding the sender to contacts

Fixing low engagement

The most effective approach is segmentation - separate your highly engaged subscribers (opened in the last 90 days) from your inactive ones (no activity in 6+ months). Send your main campaigns only to engaged subscribers. Run a separate re-engagement campaign to the inactive group. Remove anyone who doesn't respond to the re-engagement campaign.

8. Complete inbox placement checklist

9. Frequently asked questions

How long does it take to fix deliverability?

It depends on how damaged your sender reputation is. Minor issues (slightly elevated bounce rate) can recover in 2-4 weeks of clean sending. Severe issues (blacklisting, very low reputation) can take 2-3 months of consistent clean behaviour to fully recover. There is no shortcut - reputation is rebuilt through consistent good sending practices over time.

Does the "from" name affect spam filtering?

Yes, slightly. Sending from a recognisable person name like "Sarah at EmailSheriff" performs better than "noreply@" or generic names. The domain in the from address matters much more - it must match your authenticated sending domain.

Will more images make my emails go to spam?

Not inherently - but image-only emails (no text) are a strong spam signal. A healthy image-to-text ratio of around 40% images to 60% text performs best. Never send an email that is a single image with no surrounding text.

My open rate dropped suddenly. Is it spam filtering?

Not necessarily. Apple Mail Privacy Protection (launched 2021) pre-fetches all emails, which inflates open rates in Apple Mail. A sudden drop might reflect a change in your Apple Mail audience share rather than spam filtering. Check Gmail Postmaster Tools for actual domain reputation data rather than relying on open rates alone.