Email Deliverability Audit: The Complete Checklist for Diagnosing Inbox Issues

When emails stop reaching the inbox, the cause is rarely obvious. Deliverability problems are usually the result of multiple factors compounding over time: a DNS record that was never updated, a slowly degrading list, a reputation signal you stopped monitoring. An email deliverability audit is the systematic process of inspecting every factor that influences inbox placement and identifying exactly where things are breaking down.

This guide walks through a complete deliverability audit framework, organized into the five areas that matter most. Use it as a checklist whether you are troubleshooting a sudden delivery problem or conducting a proactive review.

When to Conduct a Deliverability Audit

While proactive quarterly audits are ideal, certain events should trigger an immediate review:

• A sudden drop in open rates or click rates across campaigns
• Increased bounce rates beyond your normal baseline
• Spam complaint rate spiking above 0.1%
• Being listed on one or more email blacklists
• Migrating to a new ESP, sending domain, or IP address
• Receiving delivery failures or blocks from major mailbox providers (Gmail, Outlook, Yahoo)
• Launching a new sending domain or subdomain for the first time

Part 1: Authentication Audit

Email authentication is the foundation. If your authentication records are misconfigured, incomplete, or missing, mailbox providers will treat your messages with suspicion regardless of everything else you do right.

SPF (Sender Policy Framework)

Check that your SPF record is published on every domain and subdomain you send from. Verify that it includes all IP addresses and third-party services authorized to send on your behalf. Use the SPF record checker to validate syntax and confirm it does not exceed the 10 DNS lookup limit. A common audit finding is SPF records that have accumulated stale include statements from services you no longer use.

DKIM (DomainKeys Identified Mail)

Verify that every sending source (your ESP, transactional email service, CRM, etc.) is signing messages with DKIM. Check that the DKIM public key is published in DNS and that the signature aligns with your sending domain. Test actual outbound messages to confirm signatures are valid and not being broken by intermediate processing or message modifications.

DMARC (Domain-based Message Authentication)

Confirm your DMARC policy is published and at an appropriate enforcement level. Review your DMARC aggregate reports (RUA) to identify any legitimate sending sources that are failing authentication. If you are still at p=none, your audit should include a plan and timeline for moving to p=quarantine and eventually p=reject.

# Example DMARC audit checklist items:
# 1. Is a DMARC record published? Check with: dig TXT _dmarc.yourdomain.com
# 2. What is the current policy? (none / quarantine / reject)
# 3. Are RUA reports configured and being received?
# 4. Are there legitimate sources failing DMARC in reports?
# 5. Is there a timeline to move to p=reject?

Supplementary Authentication

Check whether BIMI, MTA-STS, and TLS-RPT records are in place. While not required for basic deliverability, BIMI provides brand visibility in supporting inboxes, and MTA-STS/TLS-RPT protect against downgrade attacks on email in transit. These records signal to mailbox providers that you take email security seriously.

Part 2: Reputation Audit

Your sender reputation is the aggregate signal mailbox providers use to decide whether your emails deserve the inbox. A reputation audit checks both IP and domain reputation across all the major measurement systems.

IP Reputation

If you send on a dedicated IP, check its reputation directly. Use the Sender Reputation checker to get an overall assessment. Cross-reference with Google Postmaster Tools for Gmail-specific reputation data. Check whether your IP appears on any major blacklists using a blacklist checker.

Domain Reputation

Domain reputation has become the dominant signal for most mailbox providers, especially Google. Even on a clean IP, a domain with poor reputation will see inbox placement problems. Check your domain reputation in Google Postmaster Tools, and review your DMARC reports for any unauthorized use of your domain that could be dragging reputation down.

Blacklist Status

Scan both your sending IPs and your sending domain against all major DNSBLs (Spamhaus, Barracuda, SORBS, SpamCop, and others). A single blacklist listing can cause delivery failures at specific mailbox providers that reference that list. Document any active listings and begin the delisting process immediately.

Part 3: Infrastructure Audit

Email infrastructure issues are often overlooked because they are configured once and then forgotten. But stale DNS records, misconfigured servers, and poor IP hygiene quietly erode deliverability.

DNS Records

Verify that all email-related DNS records are correctly configured. This includes MX records for your receiving domains, A records for your sending hostnames, PTR records (reverse DNS) for your sending IPs, and that all records resolve quickly without timeouts.

Sending IP Configuration

For dedicated IPs, confirm that reverse DNS (PTR records) are properly configured with forward-confirmed reverse DNS (FCrDNS). The PTR hostname should forward-resolve back to the same IP. Also verify that the HELO/EHLO hostname your mail server announces matches the PTR record and resolves correctly.

TLS and Encryption

Verify that your mail servers support TLS and negotiate encrypted connections when sending to major providers. Most mailbox providers now flag or deprioritize messages sent without encryption. Check that your TLS certificates are valid and not expired.

Part 4: Content and Engagement Audit

Content and subscriber engagement are increasingly important ranking signals for mailbox providers. Gmail in particular uses engagement data heavily to decide inbox vs. spam placement.

Subject Lines and Preheaders

Review your recent campaigns for subject line patterns that may trigger spam filters: excessive capitalization, misleading claims, heavy use of special characters, or phrases commonly associated with spam. Preheader text should complement the subject line and encourage opens.

HTML Quality and Rendering

Poorly coded HTML, broken images, missing alt text, and excessive image-to-text ratios are all signals that spam filters evaluate. Run your email templates through rendering tests across major clients (Gmail, Outlook, Apple Mail, Yahoo) and fix any issues. Ensure a clean text-to-image ratio and include a well-formed plain-text alternative.

Engagement Metrics

Pull your open rate, click rate, and unsubscribe rate data for the last 90 days. Look for declining trends, which indicate that subscribers are losing interest and that your sending reputation may be deteriorating. Segment the data by mailbox provider if possible, as a drop at one provider (like Gmail) often points to a reputation issue specific to that provider's ecosystem.

Unsubscribe Mechanism

Verify that every email includes a working, one-click unsubscribe mechanism and a properly formatted List-Unsubscribe header. Both Google and Yahoo now require these for bulk senders. Test the unsubscribe process end to end: click the link, confirm it works, and verify the address is actually suppressed from future sends.

Part 5: List Quality Audit

Even with perfect authentication and strong infrastructure, a poor-quality list will destroy your deliverability. The list quality audit examines the health and hygiene of your subscriber database.

Bounce Rate Analysis

Review your hard bounce and soft bounce rates over the last 30-90 days. Hard bounces above 2% are a serious concern and indicate that your list contains a significant number of invalid addresses. Investigate the source of bouncing addresses: are they from a specific acquisition channel, a particular signup period, or a purchased list?

Inactive Subscriber Review

Identify subscribers who have not opened or clicked any email in 6-12 months. These chronically inactive addresses are a major drag on engagement metrics, and some may have been converted into recycled spam traps by mailbox providers. Implement a re-engagement campaign or sunset policy to either reactivate or remove these addresses.

List Acquisition Practices

Audit how email addresses are entering your list. Every acquisition channel should use either double opt-in or real-time email verification. Identify any channels that allow unverified addresses onto the list, such as event signups, partner imports, or older web forms, and add validation.

Building Your Remediation Plan

After completing all five audit sections, compile your findings into a prioritized remediation plan. Group issues by severity.

Critical (fix immediately): Missing or broken authentication records, active blacklist listings, sending without TLS, hard bounce rates above 5%.

High priority (fix within one week): SPF records exceeding lookup limits, DMARC at p=none with no progression plan, complaint rates above 0.1%, missing List-Unsubscribe headers.

Medium priority (fix within one month): Missing supplementary authentication (BIMI, MTA-STS), inactive subscriber segments not addressed, content rendering issues, incomplete engagement tracking.

Ongoing maintenance: Regular list cleaning, quarterly authentication reviews, continuous reputation monitoring, engagement trend analysis.

Frequently Asked Questions

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