Inbox vs. Spam: How DNS Configuration & Email Templates Affect Your Email Success

Understanding the core DNS records: SPF, DKIM, and DMARC

DNS (Domain Name System) records act as the internet’s address book. It is responsible for converting domain names to IP addresses and controlling different domain-related issues and settings.

For example: When it comes to email deliverability, the DNS records allow mail servers to verify if an email originated from a trusted sender. A well-configured set of DNS records (SPF, DKIM, and DMARC) minimizes the risk of spoofing, phishing attacks, and spam classification, which means your emails are more likely to land in the inbox. 

With that said, if you do not specify the right DNS records, even legitimate emails can be delivered in the spam folder. That’s also why SPF, DKIM, and DMARC are so important—they are all part of a trust mechanism between senders and email providers to ensure emails are from legitimate sources and aren’t being tampered with.

SPF (Sender Policy Framework) prevents spoofing

• SPF is a DNS record that specifies which mail servers are authorized to send emails on behalf of a domain.
• Receiving email providers then check SPF records to verify that the sending server is an authorized sender.
• Without SPF, unauthorized parties could send fraudulent emails that appear to come from your domain. Without a proper or missing SPF record, your emails are more likely to be rejected or marked as spam.

DKIM (DomainKeys Identified Mail) ensures email integrity

• DKIM appends a cryptographic signature to the email, confirming that it hasn’t been manipulated while in transit.
• When an email is sent, it is digitally signed using a private key. The recipient’s email provider checks this signature against the public DKIM key stored in the sender’s DNS records. If they match, the email is considered authentic.
• DKIM prevents attackers from modifying emails in transit and adds an extra layer of credibility to your messages. Many providers (e.g., Gmail, MS365) heavily rely on DKIM for spam filtering decisions.

DMARC (Domain-based Message Authentication, Reporting, and Conformance) enforces email security

• DMARC builds on SPF and DKIM by specifying how email providers should handle messages that fail authentication.
• A DMARC policy tells email providers whether to monitor (p=none), quarantine (p=quarantine), or reject (p=reject) emails that fail SPF and DKIM checks.
• Without DMARC, cybercriminals can spoof your domain and send phishing emails on your behalf. A properly enforced DMARC policy boosts inbox placement and email security.

The experiment: Uncovering DNS records’ impact on email deliverability

While SPF, DKIM, and DMARC improve deliverability, many senders still struggle with misconfigured DNS settings. To quantify their impact, we conducted a study testing how different configurations affect inbox vs. spam placement.

Experiment objectives:

• Validate DNS configurations: Ensure that SPF and DKIM are correctly set up across all tested domains.
• Assess DMARC implementation: Determine the current state of DMARC configuration and how it impacts email security and deliverability
• Evaluate deliverability: Analyze inbox vs. spam placement when sending emails from a Gsuite account to different providers, including Gmail, MS365, Outlook, Yahoo, Zoho, and SMTP-based services.
• Examine template impact: Investigate whether the email content type (non-HTML vs. HTML) influences inbox vs. spam placement.

The major players: New vs. Old; Configured vs. Misconfigured

We tested four distinct datasets representing different domain types and configuration to fully understand the impact of DNS configurations. Our study focused on four distinct datasets, each representing a different combination of domain age and DNS configuration.

1. New Domains (Configured DNS): Recently registered domains with correctly set SPF, DKIM, and DMARC.
2. New Domains (Misconfigured DNS): Recently registered domains with partial or incorrect DNS configurations.
3. Old Domains (Configured DNS): Older domains with properly configured DNS records.
4. Old Domains (Misconfigured DNS): Older domains with misconfigurations in SPF, DKIM, or DMARC

By testing these four categories, we aimed to answer critical questions about DNS configurations, domain age, and email deliverability and how they all tie together:

• Does a new domain with perfect DNS settings still struggle with inbox placement due to a lack of reputation?

• Can an old domain with misconfigured DNS settings still land in the inbox based on reputation alone?

• Which providers are more sensitive to DNS misconfigurations, and which ones prioritize domain reputation?

• Does proper DNS configuration always guarantee better deliverability?

Behind the scenes: how the experiment was executed

To simulate real-world email delivery conditions, we used a controlled Gsuite email account as the sender and delivered test emails to multiple recipient domains. This ensured consistency in sending conditions while allowing us to isolate the impact of DNS records.

The emails were sent to a range of email providers, each with different spam filtering policies and authentication requirements:

• Gmail (including Gsuite)
• Microsoft 365 (MS365)
• Outlook
• Yahoo
• Zoho and ZohoPro
• SMTP-based services

Tools & metrics: what we measured

To collect reliable data, we used a combination of the following tools:

• DNS lookup utilities and configuration testing tools
• Email sending simulators for sending of the test emails
• Deliverability analytics platforms to track inbox placement, spam rates, and record validation status

The metrics we tracked were:

• DNS validation: Status of SPF, DKIM, and DMARC records for each domain and if they matched expected configurations
• Deliverability outcomes: We tracked whether emails landed in the inbox or spam folders.
• Template effectiveness: Inbox placement rates for non-HTML (plain text) vs. HTML emails

• Provider-specific performance: How each email provider responded to various domain types and authentication settings.

Testing structure

• Each domain type was tested under controlled email dispatch conditions to ensure fair and unbiased comparisons. 
• Additionally, we sent a series of emails (ranging from five to 15 per test batch) from each domain category to the different providers. 
• These emails were a mixture of HTML and text-based. Specifically, we created two templates that we internally labeled as “Inbox” and “Spam” so we could gauge filter sensitivity. “Inbox” templates were those that were not deliberately selling something. They don’t contain words related to purchasing anything. Meanwhile, the “Spam” templates were templates that were heavenly customized for marketing.

The moment of truth: results and key findings

After analyzing email deliverability across different domain types, DNS configurations, and email content formats, the results revealed clear patterns in how different factors influence inbox vs. spam placement. 

Key finding #1: Proper DNS configurations lead to higher inbox rates

Emails from domains with correctly configured SPF and DKIM records were significantly more likely to land in the inbox across most providers. 

• In fact, properly configured SPF, DKIM, and DMARC domains typically achieved 70%+ inbox rates and misconfigured datasets had significantly lower rates.
• Meanwhile, misconfigured domains frequently saw spam rates above 50%—with DMARC misalignment as the culprit for the steepest drop. 

• Despite correct DNS records, new configured domains still showed lower deliverability, suggesting that there is heightened scrutiny for newly registered domains.
• In general, older domains performed better, suggesting that domain reputation also plays a big role in email deliverability.

🔖 Related research report: The Science and Process of Warming Up Newly Created Email Domains discusses how long it takes to warm up a newly created domain. This report takes a deep dive into this topic.