How do you troubleshoot email deliverability issues?

Run Mail-Tester, verify SPF/DKIM/DMARC are passing, check Google Postmaster Tools for reputation. Common causes: misconfigured authentication, spam complaint rates above 0.1%, sending to old lists, and bad shared IP reputation.

The Complete Email Deliverability Guide: From Basic to Advanced

Q: How do email authentication protocols work?

SPF tells servers which servers can send for your domain. DKIM adds a cryptographic signature verified against DNS. DMARC ties them together, tells servers what to do when authentication fails, and provides reports.

Q: What are the best practices for landing in the inbox?

Maintain list hygiene by removing bounces, optimize content with accurate subject lines, send consistently, warm up new domains gradually, honor unsubscribes immediately, and include a visible unsubscribe link.

You can craft the perfect subject line, write compelling copy, and build a gorgeous template — but none of it matters if your email never reaches the inbox.

With over 300 billion emails sent daily and roughly 20% never reaching their intended inbox, deliverability isn’t just a technical detail. It’s the foundation your entire email marketing strategy sits on.

This guide walks through everything from authentication basics to advanced monitoring strategies, so you can stop guessing and start landing in inboxes consistently.

Email deliverability journey — from send through ISP authentication checks, reputation scoring, to inbox, spam, or bounce decision

What Tools Do You Need for Email Deliverability?

Before diving into configuration, get these tools bookmarked. You’ll use them throughout this guide and for ongoing monitoring.

EasyDMARC

Your go-to for DMARC monitoring and reporting. The free tier covers most small businesses.

EasyDMARC — DMARC record generator, monitoring dashboard, and aggregate report analysis
Great for visualizing who’s sending email on your behalf

MX Toolbox

The Swiss Army knife of email diagnostics.

MX Toolbox — DNS lookups, blacklist checks, SMTP diagnostics
Use their SuperTool for quick health checks on any domain

Google Postmaster Tools

If any of your recipients use Gmail (and they do), this is essential.

Google Postmaster Tools — domain reputation, spam rate, authentication results
Free, and gives you data you literally can’t get anywhere else

Mail-Tester

Quick sanity check before any campaign goes out.

Mail-Tester — send a test email, get a score out of 10
Checks SPF, DKIM, content quality, and blacklist status in one shot

How Do Email Authentication Protocols Work?

Email authentication tells receiving servers that your emails are legitimate. Without it, you’re basically showing up to a party without an invitation — you might get in, but you probably won’t.

SPF (Sender Policy Framework)

SPF tells the world which servers are allowed to send email on behalf of your domain. It’s a DNS TXT record that lists authorized IP addresses and services.

How it works: When a server receives an email from your domain, it checks your SPF record to verify the sending server is authorized.

Example SPF record:

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

Breaking that down:

v=spf1 — version identifier (always this)
include:_spf.google.com — authorizes Google Workspace to send for you
include:sendgrid.net — authorizes SendGrid to send for you
~all — soft fail for unauthorized senders (recommended starting point)

Important limitations:

You can only have one SPF record per domain. Multiple records will cause failures.
SPF has a 10 DNS lookup limit. Each include counts as a lookup, and nested includes count too.
If you’re using multiple email services, you may need to flatten your SPF record.

DKIM (DomainKeys Identified Mail)

DKIM adds a cryptographic signature to your outgoing emails. The receiving server can verify that signature against a public key in your DNS, confirming the email wasn’t tampered with in transit.

How it works: Your email server signs each outgoing message with a private key. The corresponding public key lives in your DNS as a TXT record.

Example DKIM DNS record:

selector._domainkey.yourdomain.com  TXT  v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEB...

Key points:

The selector is provided by your email service (e.g., google, s1, k1)
The p= value is your public key — your email provider generates this
Each sending service needs its own DKIM selector

DMARC (Domain-based Message Authentication, Reporting & Conformance)

DMARC ties SPF and DKIM together and tells receiving servers what to do when authentication fails. It also gives you reporting on who’s sending email as your domain.

Start with monitoring mode:

v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.com; pct=100

p=none — don’t take action on failures, just report (start here)
rua=mailto:... — where to send aggregate reports
pct=100 — apply the policy to 100% of messages

Progress to enforcement over time:

# After 2-4 weeks of clean reports:
v=DMARC1; p=quarantine; rua=mailto:dmarc@yourdomain.com; pct=100

# After another 2-4 weeks with no issues:
v=DMARC1; p=reject; rua=mailto:dmarc@yourdomain.com; pct=100

The progression matters. Going straight to p=reject without monitoring first is how you accidentally block your own legitimate email.

Platform-Specific Setup

GoDaddy DNS

Log in to your GoDaddy account and navigate to My Products > DNS
For SPF: Add a TXT record with @ as the host and your SPF string as the value
For DKIM: Add a TXT record with selector._domainkey as the host (replace selector with your provider’s value)
For DMARC: Add a TXT record with _dmarc as the host
TTL: Start with 600 seconds during setup, increase to 3600 once verified

Microsoft 365

Microsoft 365 handles DKIM through the admin center:

Go to Admin Center > Settings > Domains
Select your domain and click DNS Records
Microsoft provides two CNAME records for DKIM — add both to your DNS
Back in the admin center, enable DKIM signing for your domain
SPF for Microsoft 365: include:spf.protection.outlook.com

Google Workspace

Admin Console > Apps > Google Workspace > Gmail > Authenticate Email
Click Generate New Record — Google gives you the DKIM TXT record
Add it to your DNS, then click Start Authentication in the admin console
SPF for Google Workspace: include:_spf.google.com
Allow 24-48 hours for DNS propagation before testing

What Are the Best Practices for Landing in the Inbox?

List Hygiene

Your list is a living thing. Treat it that way.

Remove hard bounces immediately. No second chances. A hard bounce means the address doesn’t exist.
Suppress soft bounces after 3-5 attempts. The mailbox might be full or temporarily unavailable, but persistent soft bounces become a problem.
Run re-engagement campaigns for subscribers who haven’t opened or clicked in 90 days. If they still don’t engage, remove them.
Never purchase email lists. Ever. The short-term gain isn’t worth the long-term damage to your sender reputation.
Use confirmed opt-in (double opt-in) for new subscribers. It adds friction, but the list quality improvement is worth it.

Content Optimization

Write subject lines that accurately reflect the email content. Clickbait hurts your reputation.
Maintain a healthy text-to-image ratio. All-image emails are a spam signal.
Avoid spam trigger words in subject lines — “FREE!!!”, “Act Now”, “Limited Time” used excessively
Include a plain-text version of every HTML email
Keep your HTML clean. Broken tags and inline styles copied from Word are red flags.

Sending Practices

Warm up new domains and IPs gradually. Start with your most engaged subscribers, send small volumes, and increase over 2-4 weeks.
Send consistently. Erratic volume is suspicious. If you normally send 5,000 emails per week, don’t suddenly send 50,000.
Respect time zones. Sending at 3 AM local time isn’t just rude — it increases the chance of being marked as spam.
Honor unsubscribes immediately. Not “within 10 business days.” Immediately.
Include a visible, easy-to-find unsubscribe link. Making it hard to unsubscribe means people hit the spam button instead, which is far worse for you.

Monitoring and Maintenance Schedule

Daily (5 minutes)

Check bounce rates on any campaigns sent in the last 24 hours
Review spam complaint rates in Google Postmaster Tools
Glance at delivery rates — any sudden drops need immediate attention

Weekly (15 minutes)

Review DMARC aggregate reports for unauthorized senders
Check blacklist status via MX Toolbox
Review engagement metrics (opens, clicks, unsubscribes) for trends
Process any new hard bounces or spam complaints

Monthly (30 minutes)

Run a full Mail-Tester check on a test email
Audit your subscriber list — remove disengaged contacts
Review sending volume trends
Check that all DNS records (SPF, DKIM, DMARC) are still correctly configured
Review Google Postmaster Tools domain reputation trend

How Do You Troubleshoot Deliverability Issues?

Emails Going to Spam

Check first:

Run Mail-Tester — it will flag the most common issues
Verify SPF, DKIM, and DMARC are all passing (use MX Toolbox)
Check Google Postmaster Tools for reputation issues

Common causes:

Missing or misconfigured authentication records
High spam complaint rate (above 0.1% is a problem)
Sending to old, unengaged lists
Content that triggers spam filters
Shared IP address with a bad reputation (common on budget email platforms)

Sudden Drop in Delivery Rates

Check if you’ve been added to a blacklist (MX Toolbox blacklist check)
Review recent changes to DNS records — did someone accidentally modify your SPF?
Look for a spike in bounce rates or spam complaints
Check if your email service provider made infrastructure changes

Authentication Failures

SPF failing: Verify you haven’t exceeded the 10 DNS lookup limit. Check that all sending services are included.
DKIM failing: Confirm the selector matches what your email service expects. Check for DNS propagation delays.
DMARC failing: If SPF and DKIM both pass but DMARC fails, you likely have an alignment issue — the “From” domain must match the SPF/DKIM domain.

Advanced Strategies

BIMI (Brand Indicators for Message Identification)

BIMI lets you display your brand logo next to your emails in supporting inboxes (Gmail, Apple Mail, Yahoo). It requires a DMARC policy of quarantine or reject, plus a Verified Mark Certificate (VMC) from a certificate authority.

Setup steps:

Achieve DMARC enforcement (p=quarantine or p=reject)
Create an SVG version of your logo that meets BIMI specifications
Obtain a VMC from DigiCert or Entrust
Add a BIMI DNS record: default._bimi.yourdomain.com

BIMI is a longer-term play, but it significantly increases brand recognition and trust in the inbox.

AI-Powered Optimization

Modern email platforms are incorporating AI for:

Send time optimization — analyzing individual recipient behavior to send at the moment they’re most likely to engage
Subject line testing — generating and testing variations at scale, beyond what manual A/B testing can cover
Content personalization — dynamically adjusting email content based on recipient behavior and preferences
Predictive list management — identifying subscribers likely to disengage before they do, so you can re-engage proactively

Resources

Google Postmaster Tools — free Gmail deliverability data
MX Toolbox — DNS diagnostics and blacklist monitoring
EasyDMARC — DMARC deployment and monitoring
Mail-Tester — email scoring and diagnostics
DMARC.org — official DMARC specification and resources
SPF Record Syntax — detailed SPF reference

Success Metrics: What Good Looks Like

Track these KPIs to know your deliverability program is working:

Metric	Target	Warning Threshold
Delivery Rate	> 95%	< 90%
Bounce Rate	< 2%	> 5%
Spam Complaint Rate	< 0.1%	> 0.3%
Open Rate	20-30% (varies by industry)	< 15%
Unsubscribe Rate	< 0.5%	> 1%
DMARC Pass Rate	> 98%	< 95%

These are industry benchmarks — your specific numbers will vary by audience and sector. The important thing is trending in the right direction.

Implementation Checklist

Use this to track your progress:

Email deliverability isn’t a set-it-and-forget-it situation. It’s an ongoing practice. But with the right foundation and a consistent monitoring routine, you can keep your emails landing where they belong — in the inbox. Deliverability is the foundation of any effective email marketing strategy — and it connects directly to your broader AI-powered marketing system when every touchpoint is tracked. Your analytics and reporting should include deliverability metrics alongside campaign performance so you can spot problems before they cost you revenue.

Want to see how your digital presence performs overall? Run a free AI Visibility scan — 22 checks, 5 pillars, results in 30 seconds.