How to Troubleshoot Deliverability Issues Like a Pro

Email is the backbone of outbound sales and startup growth. But what’s the use of a perfectly crafted cold email if it never reaches the recipient’s inbox? For startups and sales teams, email deliverability isn’t just a technical metric, it’s a direct driver of pipeline, partnerships, and revenue.
Yet, even the most seasoned teams run into deliverability problems: emails landing in spam, sudden drops in open rates, or outright blocks from major providers. These issues can feel overwhelming, but with the right troubleshooting process, you can quickly diagnose the root cause, restore your sender reputation, and get back to scaling your outreach.
This guide walks you through a comprehensive, step-by-step approach to identifying, diagnosing, and resolving deliverability issues. You’ll learn how to spot common pitfalls, leverage key tools, and implement best practices, so your emails land where they belong.

Why Deliverability Matters

Before diving into troubleshooting, it’s crucial to understand why deliverability deserves your attention:

• Every missed inbox is a missed opportunity. If your emails aren’t seen, your prospecting, nurturing, and deal-closing efforts all take a hit.
• Deliverability impacts your sender reputation. Poor practices can create a negative feedback loop, making it even harder to reach your audience over time.
• The stakes are higher for startups and sales teams. You’re often sending cold emails at scale, relying on high open rates, and working with lean resources.

Understanding Email Deliverability

Email deliverability is the ability of your emails to reach the recipient’s primary inbox, rather than being filtered to spam, junk, or blocked entirely. Deliverability is influenced by a combination of technical, content, and behavioral factors.

Key Factors Affecting Deliverability

1. Authentication:
   
   • DNS records (SPF, DKIM, DMARC) verify you as a legitimate sender.
   • Missing or misconfigured records can cause ISPs to distrust your messages.
2. Sender Reputation:
   
   • ISPs track your sending history, engagement rates, bounce rates, and spam complaints.
   • A poor reputation can send your emails straight to spam—or block them altogether.
3. Content Quality:
   
   • Spammy language, excessive links, poor formatting, or attachments can trigger filters.
4. Engagement:
   
   • High open, reply, and click rates signal trustworthiness to email providers.
   • Low engagement can reduce inbox placement over time.

The Most Common Causes of Deliverability Issues

Even with the best intentions, many teams fall into these traps:

1. DNS Record Misconfigurations

• SPF (Sender Policy Framework): Ensures only authorized servers can send emails on your domain’s behalf.
• DKIM (DomainKeys Identified Mail): Adds a digital signature to your emails, proving authenticity.
• DMARC (Domain-based Message Authentication, Reporting, and Conformance): Sets a policy for how ISPs should handle unauthenticated emails.

Common problems:

• Records are missing or not published to DNS
• Syntax errors in DNS entries
• Misalignment between the “from” domain and the sending domain

2. Poor Sender Reputation

• High bounce rates (sending to invalid addresses)
• Spam complaints from recipients
• Sudden spikes in sending volume
• Sending from newly registered domains without proper warm-up

3. High Bounce Rates

• Outdated or purchased lists
• No regular list cleaning
• Ignoring hard bounces (permanently undeliverable addresses)

4. Blacklists and Blocklists

• Your sending IP or domain appears on public blacklists (e.g., Spamhaus, Barracuda)
• Blacklisting can happen due to spam complaints, malware, or compromised accounts

5. Content Triggers

• Use of spammy phrases (“Buy now!”, “Free!”, “Act now!”)
• Excessive capitalization or punctuation (!!!)
• Too many links or images
• Attachments, especially executable files

Step-by-Step Troubleshooting Process

Ready to fix your deliverability? Here’s a battle-tested process:

Step 1: Diagnose the Problem

Start by gathering evidence:

• Check bounce messages:
  Review SMTP error codes and bounce notifications. Are you seeing “550” errors (blocked), “554” (spam detected), or “421” (temporary failure)?
• Inbox placement testing:
  Use tools like GlockApps or Mail-Tester to see where your emails land (inbox, spam, or promotions).
• Monitor open and reply rates:
  A sudden drop may signal deliverability issues.

Step 2: Check DNS Records

• SPF:
  Use tools like MXToolbox or Google Admin Toolbox to verify your SPF record is present and correct.
• DKIM:
  Ensure DKIM is enabled and the public key is published in DNS.
• DMARC:
  Set up a DMARC policy (at minimum, “none” for monitoring) and monitor reports for authentication failures.

Step 3: Review Sender Reputation

• Google Postmaster Tools:
  Check your domain’s reputation, IP reputation, and feedback loop complaints.
• Microsoft SNDS:
  For Outlook delivery, monitor your IP status and complaint rates.
• Third-party tools:
  Platforms like Mailpool, Talos Intelligence, or SenderScore can provide reputation insights.

Step 4: Audit Recent Campaigns and Content

• Look for spikes in volume:
  Did you recently increase sending? Sudden jumps can trigger blocks.
• Check for spammy content:
  Review subject lines, body copy, and links for potential triggers.
• Analyze recipient engagement:
  Are your lists active, or are you sending to unengaged users?

Step 5: Monitor Blacklists and Feedback Loops

• Blacklist checks:
  Use tools like MXToolbox, MultiRBL, or Blacklist Check to see if your domain or IP is listed.
• ISP feedback loops:
  Sign up for complaint feedback loops with major ISPs to receive reports and take action on complaints.

How to Restore and Protect Sender Reputation

If you’ve detected issues, here’s how to get back on track and stay there:

1. Warm Up New Domains and IPs

• Start with a small volume of emails per day
• Gradually increase volume over several weeks
• Mix in personal, one-to-one emails to build trust

2. Manage Sending Volume and Frequency

• Avoid sudden spikes in sending
• Stick to a consistent schedule
• Monitor daily and hourly sending limits

3. Maintain List Hygiene

• Regularly remove inactive, bounced, or unengaged addresses
• Never buy or scrape email lists
• Use double opt-in where possible

4. Boost Engagement

• Personalize subject lines and content
• Segment lists for relevance
• Encourage replies and interaction

5. Monitor Ongoing Reputation

• Use deliverability tools to track inbox placement and reputation scores
• Set up alerts for blacklisting or sudden changes in metrics

Advanced Troubleshooting

Quick Fixes for Urgent Issues

• Pause sending from affected domains or IPs:
  Continuing to send can worsen your reputation.
• Switch to a backup domain or IP:
  If available, use a healthy sending identity while troubleshooting.
• Remove problematic content:
  Edit or remove emails that triggered blocks.

When to Reach Out to Providers

• If you’re blocked or blacklisted:
  Contact the ISP or blacklist provider directly. Provide evidence of remediation (e.g., cleaned list, fixed DNS).
• Document your process:
  Keep records of changes made and communications sent.

Leverage Deliverability Platforms

• Tools like Mailpool automate DNS setup, monitor inbox placement, and provide actionable alerts.
• Use built-in analytics to spot trends and address issues before they escalate.

Preventing Future Deliverability Problems

Deliverability isn’t a one-time fix; it’s an ongoing process. Here’s how to future-proof your outreach:

• Continuously monitor authentication records:
  Any DNS changes can affect SPF, DKIM, or DMARC.
• Regularly clean your lists:
  Set a schedule for list maintenance.
• Educate your team:
  Make sure everyone understands best practices for sending and content.
• Stay up to date:
  Email providers update their algorithms; keep learning and adapting.

Conclusion

Email deliverability is a critical pillar of outbound success for startups and sales teams. By understanding the technical and behavioral factors at play, proactively monitoring your reputation, and acting quickly to resolve issues, you can keep your outreach effective and your pipeline full.
Don’t wait for problems to escalate. Make deliverability a core part of your sales and marketing operations. And if you need expert help, specialized tools can make all the difference.
Ready to take your deliverability to the next level? Book a demo with Mailpool and see how we can help you troubleshoot, optimize, and scale your email campaigns with confidence.