Email deliverability: why your emails land in spam and how to fix it

Your email was delivered. It just never reached the inbox.

Most marketers only discover this after weeks of declining engagement, assuming their subject lines or content are the problem. They're optimising the wrong thing. Email deliverability, whether your messages actually reach the inbox, is the hidden variable that determines whether your program works at all.

A delivered email is not the same as an email in the inbox. Your ESP reports a 98% delivery rate, but half your list never sees your messages because they landed in spam. You can't open what you never see.

The metric that actually matters: inbox placement rate

Open rate tells you what percentage of people opened your email. It doesn't tell you how many people had the chance to open it in the first place.

Inbox placement rate is the percentage of your emails that reach the primary inbox versus spam, promotions tabs, or other filtered locations. For most senders, this number sits between 70–95%. A 10-point drop means thousands of subscribers never saw your message.

The problem: most email platforms don't report inbox placement by default. You're flying blind unless you actively monitor it.

Gmail processes over 300 billion emails daily. Their filtering algorithms don't care about your clever copy. They care about three things: whether you're authenticated, whether recipients engage with your mail, and whether your list is clean. Get those wrong and you're in spam, regardless of content quality.

The three pillars of email deliverability

Every deliverability problem traces back to one of three areas. Fix these systematically and you fix inbox placement.

Authentication: proving you are who you say you are

SPF, DKIM, and DMARC are the authentication protocols that verify your emails actually come from your domain. Without them, mailbox providers assume you're a spammer or a phishing attempt.

SPF (Sender Policy Framework) lists which servers are authorised to send email on your behalf. Your ESP's IP addresses need to be in this list, published as a DNS record on your domain.

DKIM (DomainKeys Identified Mail) adds a cryptographic signature to every email you send. The receiving server checks this signature against a public key in your DNS. If they match, the email hasn't been tampered with in transit.

DMARC (Domain-based Message Authentication) tells receiving servers what to do if SPF or DKIM checks fail: quarantine the message, reject it, or do nothing. It also generates reports showing you who's trying to send email using your domain.

Most authentication problems are configuration errors. Your SPF record includes an old ESP you're no longer using. Your DKIM signature isn't aligned with your From address. Your DMARC policy is set to "none" so it's not actually enforced.

Run the free email program audit to check your authentication status. If anything's misconfigured, it'll show up immediately.

Sender reputation: your track record with mailbox providers

Authentication proves you're legitimate. Sender reputation proves you're wanted.

Every IP address and domain that sends email accumulates a reputation score with major mailbox providers. High engagement (opens, clicks, replies) builds reputation. High complaint rates, spam trap hits, and low engagement destroy it.

Gmail doesn't publish their reputation algorithm, but the inputs are observable:

• Complaint rate above 0.3% and you're in trouble
• Engagement rate below industry benchmarks and you're filtered
• Sudden volume spikes and you're flagged as suspicious
• Sending to invalid or inactive addresses and you're marked as low-quality

Your reputation follows both your sending IP and your domain. Switch ESPs and you might escape a bad IP reputation, but your domain reputation comes with you.

The practical implication: you can't buy your way out of a reputation problem. Sending more emails when your reputation is damaged makes it worse. The only fix is to reduce volume, clean your list, and let engagement rebuild over weeks.

List hygiene: removing the addresses that hurt you

An email address that never opens your messages is not a neutral asset. It's actively damaging your deliverability.

Mailbox providers interpret consistent non-engagement as a signal that recipients don't want your mail. Send to enough inactive addresses and your reputation score drops, which affects deliverability to your engaged subscribers too.

List hygiene means systematically removing:

• Invalid addresses (hard bounces)
• Spam traps (email addresses created specifically to catch bad senders)
• Inactive subscribers (no opens or clicks in 6–12 months)
• Role addresses that never engage (info@, sales@, support@)

Most programs have 15–30% inactive subscribers. These addresses cost money to store and send to, while simultaneously degrading your sender reputation. Suppressing them improves deliverability to everyone else.

The counterintuitive move: send less email to a smaller list and you'll reach more people.

How to diagnose a deliverability problem

Deliverability issues show up as engagement drops. The question is whether it's a content problem or an inbox placement problem.

Start with seed testing. Set up test accounts on Gmail, Outlook, Yahoo, and any other major provider your audience uses. Send your campaign and check where it lands: primary inbox, promotions tab, or spam. If more than 10% land in spam, you have a deliverability issue.

Check your complaint rate. Every ESP reports this, it's the percentage of recipients who marked your email as spam. Anything above 0.1% is concerning. Above 0.3% and you're being filtered aggressively.

Review your bounce rate. Hard bounces (permanent delivery failures) should be under 2%. If you're seeing 5% or higher, your list contains old or purchased data, and you're hitting spam traps.

Monitor engagement trends over 90 days. A steady decline in open rates across all campaigns suggests inbox placement is dropping. A sudden drop in a single campaign suggests content or subject line issues.

The most reliable diagnostic tool is Google Postmaster Tools, which shows you exactly how Gmail treats your mail.

Google Postmaster Tools: your window into Gmail's filtering

Google Postmaster Tools is a free service that reports your sender reputation, spam rate, and inbox placement for Gmail specifically. Given Gmail's 1.8 billion users, this data matters.

Set it up by verifying your sending domain via DNS. Within a few days, you'll see four critical metrics:

Spam rate shows what percentage of your emails were marked as spam by recipients. Anything above "low" (under 0.1%) means you're being filtered more aggressively. If you're in the "high" category, most of your mail is going to spam by default.

IP reputation and domain reputation are shown as colour-coded bars: high (green), medium (yellow), or low/bad (red). Medium or low reputation means your inbox placement is compromised. Bad reputation means you're almost entirely in spam.

Inbox placement rate (when available) shows what percentage of your delivered emails actually reached the inbox versus spam or other folders. This is the metric that directly correlates with your open rate. A 50% inbox placement rate means half your list never had a chance to see your message.

Authentication status confirms whether SPF, DKIM, and DMARC are passing. If anything shows red here, fix it immediately, you're being treated as unauthenticated, which means aggressive filtering.

Most deliverability problems show up in Postmaster Tools 1–2 weeks before they appear in your campaign metrics. Monitor it weekly and you can fix issues before they crater your engagement.

Fixing deliverability: the systematic approach

You can't fix everything at once. Prioritise based on severity.

If authentication is failing, fix it this week. SPF, DKIM, and DMARC are table stakes. No other improvement matters if you're not authenticated. Work with your IT team or DNS provider to get the records correct.

If your complaint rate is above 0.3%, stop all non-essential sends immediately. Review your list for people who haven't engaged in 6+ months and suppress them. Send only to your most engaged segment until your reputation recovers.

If your list has inactive subscribers (no engagement in 6–12 months), run a re-engagement campaign. Send a "we'll miss you" message giving them one last chance to confirm they want your emails. Suppress everyone who doesn't respond. Yes, this will shrink your list. That's the point.

If you're seeing low engagement across all campaigns, consider your frequency. Many programs send too often, training subscribers to ignore their messages. Test sending less frequently to your full list and more frequently only to engaged segments.

Reputation recovery takes 4–8 weeks minimum. You're demonstrating a new pattern of behaviour to mailbox providers. Consistency matters more than any single tactic.

When to bring in help

Most deliverability problems can be fixed in-house if you know what you're looking for. Authentication is straightforward. List hygiene is tedious but simple. Reputation recovery just takes time.

The exception: if you're consistently hitting spam despite proper authentication, a clean list, and engaged subscribers, you likely have a technical issue that requires specialist diagnosis. IP blacklisting, DMARC policy conflicts, or infrastructure problems need expert attention.

If you've fixed the obvious issues and inbox placement is still poor, get in touch. We'll run a technical audit and identify exactly what's blocking you.

Your emails need to reach the inbox before anything else matters. Fix deliverability first. Optimise content second.