Email Deliverability Explained: Why Your Emails Land in Spam

What Deliverability Actually Means

When you send an email, there are three possible outcomes. It lands in the inbox. It lands in the spam or junk folder. Or it disappears entirely — rejected by the receiving server before it even gets classified. Deliverability measures the first outcome: what percentage of your sent emails actually reach the inbox.

This is different from delivery rate, which most email platforms report. Delivery rate just means the email was accepted by the receiving server — it says nothing about whether it went to inbox or spam. A 98% delivery rate sounds great until you realize 30% of those delivered emails went straight to junk.

The distinction matters because spam folder placement is nearly invisible. Your subscribers will not tell you they are not seeing your emails — they will just stop engaging. Open rates quietly decline. Click rates drop. Revenue from email flatlines. And because email platforms report delivery rate rather than inbox placement rate, you may not realize there is a problem for months.

Industry-wide, average inbox placement rates hover around 83-87% according to various deliverability monitoring services. That means roughly 1 in 7 marketing emails never reaches the inbox. For a list of 10,000 subscribers, that is 1,300-1,700 people who simply do not see your message.

Authentication Basics: SPF, DKIM, and DMARC

Email authentication is the foundation of deliverability. Without it, inbox providers like Gmail, Outlook, and Yahoo have no way to verify that you are who you claim to be — and they will treat your emails accordingly.

SPF (Sender Policy Framework) is a DNS record that tells receiving servers which mail servers are authorized to send email on behalf of your domain. When you sign up for an email platform like MailerLite or ActiveCampaign, they give you an SPF record to add to your domain's DNS settings. Without it, your emails lack a basic identity check.

DKIM (DomainKeys Identified Mail) adds a digital signature to every email you send. The receiving server checks this signature against a public key published in your DNS. If the signature matches, the email has not been tampered with in transit. Think of it as a seal of authenticity.

DMARC (Domain-based Message Authentication, Reporting and Conformance) ties SPF and DKIM together and tells receiving servers what to do when authentication fails — deliver the email anyway, quarantine it, or reject it outright. DMARC also sends you reports about who is sending email using your domain, which helps you detect spoofing.

As of February 2024, Gmail and Yahoo require bulk senders (over 5,000 emails per day) to have all three — SPF, DKIM, and DMARC — properly configured. This is no longer optional for high-volume senders. Even at lower volumes, having all three dramatically improves your inbox placement.

Every reputable email platform provides setup guides for these records. The process takes about 15 minutes and involves adding 2-3 DNS records through your domain registrar. If your email platform has not asked you to do this, or if you skipped the step, your deliverability is almost certainly suffering.

Your Sender Reputation

ISPs (Gmail, Outlook, Yahoo, and others) assign a reputation score to every sender. This score is based on your domain reputation and your IP reputation. Both affect whether your emails reach the inbox.

Domain reputation is tied to the domain you send from. It is built over time based on engagement metrics — how often people open, click, and reply to your emails versus how often they mark you as spam, ignore you, or unsubscribe. A new domain starts with no reputation, which is why deliverability often dips when you start sending from a new domain or switch platforms.

IP reputation is tied to the mail server's IP address. If you are on a shared sending plan (which most small-to-mid senders are), you share an IP with other customers of your email platform. Their behavior affects your deliverability. If someone on the same IP is sending spammy content, it can drag your inbox placement down. This is one reason premium platforms like ActiveCampaign offer dedicated IPs for high-volume senders — you control your own reputation.

Google Postmaster Tools is a free service that shows you your domain reputation with Gmail specifically. It rates you as High, Medium, Low, or Bad. If you send any volume of email, you should be monitoring this. Yahoo has a similar tool called Sender Hub.

Reputation takes weeks to build and moments to destroy. A single campaign sent to a poorly maintained list — full of inactive addresses, spam traps, and invalid emails — can tank your sender score and take months to recover.

Content That Triggers Spam Filters

Modern spam filters use machine learning and are far more sophisticated than the keyword-matching systems of a decade ago. That said, certain content patterns still increase your spam risk.

Excessive use of sales language — FREE, ACT NOW, LIMITED TIME, BUY NOW — in subject lines still triggers some filters, particularly when combined with excessive capitalization and exclamation marks. A subject line like "FREE!!! ACT NOW!!!" is essentially a spam filter test case.

Image-heavy emails with little text are a red flag. Spammers historically used images to bypass text-based filters, so emails that are mostly images with minimal readable text get scrutinized. Aim for at least a 60/40 text-to-image ratio.

Link density matters. An email with more links than text paragraphs looks suspicious. Shortened URLs (bit.ly, tinyurl) are frequently associated with spam and phishing — use full URLs or your own branded link shortener.

Missing unsubscribe links are a compliance issue and a spam filter trigger. Every marketing email must have a clear, functional unsubscribe mechanism. As of 2024, Gmail requires one-click unsubscribe headers for bulk senders.

Sending patterns also matter. Sudden volume spikes — going from 100 emails per week to 10,000 — trigger fraud detection systems. If you are growing your list quickly or migrating platforms, ramp up your sending volume gradually over 2-4 weeks. This is called IP warming and is critical when starting with a new platform.

The most reliable content advice: write emails you would want to receive. If your email looks like something a legitimate business would send to an engaged customer, modern spam filters will treat it as such.

How Email Platforms Affect Deliverability

Your choice of email platform directly influences your baseline deliverability. Some platforms maintain stricter sending policies, invest more in deliverability infrastructure, and are more aggressive about removing bad actors from their shared IPs.

EmailToolTester has been running independent deliverability tests since 2015, averaging results across multiple rounds to smooth out variability. Their recent data shows meaningful differences between platforms.

MailerLite consistently leads with a 94.41% average inbox placement rate across their test rounds. Their strict account approval process — which frustrates some users — is a deliberate trade-off: by rejecting senders who might harm deliverability, they keep the shared IP pool cleaner.

ActiveCampaign averages around 89.6% inbox placement. Their infrastructure is well-maintained, and they offer dedicated IPs for higher-volume senders who want full control over their reputation.

Mailchimp comes in at approximately 87%, which is decent but not leading. Given their enormous user base (over 12 million accounts), maintaining deliverability at scale is a genuine challenge.

Brevo averages approximately 85%, and GetResponse is similar at around 85.5%. Both are adequate for most use cases but trail the leaders by a noticeable margin.

These numbers are averages — your individual results will vary based on your content, list quality, and sending practices. But they establish a baseline. Choosing a platform with strong deliverability infrastructure gives you a head start.

List Hygiene Practices

Your subscriber list degrades over time. People abandon email addresses. Inboxes fill up. Domains expire. On average, email lists decay at roughly 22-30% per year according to various email marketing research studies. If you are not actively maintaining your list, a growing percentage of your sends are going to dead addresses.

Invalid addresses hurt your bounce rate. Hard bounces (permanent delivery failures) signal to ISPs that you are not maintaining your list, which damages your sender reputation. Keep your hard bounce rate below 2% — most platforms will suspend your account if it consistently exceeds that threshold.

Spam traps are recycled email addresses that ISPs and anti-spam organizations use to catch senders who do not maintain their lists. If a subscriber has not engaged with your emails in 12-24 months, that address may have been converted to a spam trap. Sending to it is a direct hit to your reputation.

Practical list hygiene steps: remove hard bounces immediately (most platforms do this automatically). Remove soft bounces after 3-5 consecutive failures. Create a re-engagement campaign for subscribers who have not opened or clicked in 90 days. If they still do not engage after 2-3 re-engagement attempts, remove them. Yes, your list will shrink. Your deliverability and engagement rates will improve, and you will stop paying to email people who are not listening.

Consider running your list through a verification service (ZeroBounce, NeverBounce, or BriteVerify) annually, or before migrating to a new platform. These services identify invalid, risky, and disposable addresses before they damage your reputation.

How to Diagnose Deliverability Problems

If your open rates have been declining steadily over several months, deliverability might be the culprit. Here is how to investigate.

First, check Google Postmaster Tools. If your domain reputation is Medium or Low with Gmail, you have a deliverability problem. Look at the spam rate — if more than 0.1% of Gmail recipients are marking your emails as spam, that is a warning sign. Above 0.3% and you are in dangerous territory.

Second, send test emails to seed accounts at Gmail, Outlook, and Yahoo. Check whether they land in inbox or spam. Tools like Mail-Tester.com give you a free score based on your email content, authentication, and blacklist status.

Third, check blacklists. MXToolbox has a free blacklist checker that scans your domain and sending IP against major spam blacklists. If you are listed on any, that is a direct cause of deliverability issues.

Fourth, review your bounce rate trends. An increasing bounce rate over time indicates list decay. A sudden spike in bounces suggests you may have imported bad data or your list has not been cleaned in too long.

Fifth, compare open rates across inbox providers if your platform provides this data. If Gmail open rates are significantly lower than Outlook, you likely have a Gmail-specific deliverability issue, often related to spam reports or domain reputation with Google specifically.

Once you have identified the problem, the fix usually involves some combination of: cleaning your list aggressively, improving authentication, adjusting your content, and temporarily reducing your sending volume to rebuild reputation. If you are on shared IPs and suspect the platform's infrastructure is the issue, consider switching to a platform with better deliverability scores — or request a dedicated IP if your volume justifies it.