Module 2 · Chapter 8

Monitoring and fixing deliverability issues

9 min read

Deliverability is not a one-time setup. Email provider algorithms change, blacklists update, domain reputation fluctuates, and sending patterns evolve. Without ongoing monitoring, problems can silently destroy your campaigns for days or weeks before you notice the drop in replies.

This chapter covers the tools, metrics, and processes you need to monitor deliverability continuously, diagnose issues quickly, and remediate problems before they become permanent.

Essential monitoring tools

Google Postmaster Tools

If your prospects use Gmail or Google Workspace (and most B2B contacts do), Google Postmaster Tools is the single most important monitoring tool. It provides direct data from Google about how they see your sending domain.

What it shows:

  • Domain reputation: Rated as High, Medium, Low, or Bad. You want Medium or High. Low means deliverability problems. Bad means most emails are going to spam.
  • Spam rate: The percentage of your emails that Gmail users mark as spam. Must stay below 0.3% to maintain good standing. Below 0.1% is ideal.
  • Authentication: SPF, DKIM, and DMARC pass rates. Should all be near 100%.
  • Encryption: Percentage of emails sent over TLS. Should be 100% with modern email providers.

Setup: Go to postmaster.google.com, add your domain, and verify ownership by adding a DNS TXT record. Data appears once you have sufficient sending volume (typically 100+ emails to Gmail per day).

MXToolbox

MXToolbox is a free tool that checks your domain against multiple blacklists, verifies DNS records, and tests email server configuration. Use it for:

  • Blacklist monitoring: Checks your domain and IP against 100+ blacklists simultaneously
  • DNS record verification: Confirms SPF, DKIM, DMARC, and MX records are correctly configured
  • SMTP diagnostics: Tests your mail server's ability to send and receive email

Set up free MXToolbox alerts to monitor your domains automatically. You will receive an email if your domain appears on any blacklist.

mail-tester.com

Mail-tester gives you a deliverability score out of 10 for any individual email. Send a test email to the address they provide, and they analyze everything: authentication, content, blacklists, formatting.

Run a mail-tester check:

  • Before launching any new campaign
  • After making DNS changes
  • When you notice open rates dropping
  • Weekly as a routine check

Target: 9/10 or higher. Anything below 7 indicates serious issues that need immediate attention.

Your outreach platform's built-in analytics

Most outreach platforms track deliverability metrics per inbox and per campaign. Pay attention to:

  • Bounce rate per inbox (target: under 2%)
  • Open rate trends per inbox (sudden drops signal deliverability problems)
  • Warm-up inbox placement rate (should stay above 90%)

What to monitor and how often

Metric Frequency Target Action threshold
Bounce rate Daily <2% >3% = pause and investigate
Open rate Daily >40% <25% = likely spam folder issue
Spam complaints Daily <0.1% >0.3% = urgent
Blacklist status Weekly Not listed Any listing = investigate
Domain reputation Weekly Medium/High Low = reduce volume
Authentication pass rate Weekly 100% <95% = check DNS

Diagnosing common issues

Symptom: Open rates suddenly dropped

If open rates drop by more than 30% compared to your baseline, your emails are likely landing in spam or the Promotions tab.

Diagnosis steps:

  1. Send a test email to a personal Gmail account. Check if it lands in Primary, Promotions, or Spam.
  2. Run a mail-tester.com check. Look for any new warnings.
  3. Check MXToolbox for blacklist status.
  4. Review Google Postmaster Tools for reputation changes.
  5. Check if you recently changed your email content, added links, or increased volume.

Symptom: High bounce rate

Diagnosis steps:

  1. Check if bounces are hard bounces (invalid addresses) or soft bounces (temporary issues).
  2. For hard bounces: your email verification process has gaps. Re-verify your list with a different verification tool.
  3. For soft bounces concentrated on one domain: the recipient's server may be blocking you. Check blacklists.
  4. For soft bounces across many domains: your sending IP or domain may have reputation issues.

Symptom: Blacklisted domain

Diagnosis steps:

  1. Identify which blacklist you are on (MXToolbox shows this).
  2. Determine the cause: high bounce rates, spam complaints, or malware (unlikely but possible if your account was compromised).
  3. Fix the root cause before requesting delisting.
  4. Submit a delisting request through the blacklist's website. Most have automated processes.
  5. Monitor to ensure you are not re-listed after delisting.

Key insight

Not all blacklists are equally important. Spamhaus, Barracuda, and SpamCop are the most impactful. Being listed on a minor blacklist that nobody checks is annoying but not critical. Prioritize delisting from the major ones.

The remediation playbook

When deliverability goes wrong, follow this structured plan:

Step 1: Stop the bleeding

Immediately reduce or pause sending from the affected inbox or domain. Continuing to send while reputation is tanking only makes things worse. If one inbox or domain is affected, shift volume to healthy ones.

Step 2: Diagnose the root cause

Use the tools and steps outlined above. The most common causes are: spiked sending volume, bad list quality (high bounces), content changes that triggered filters, or a blacklist appearance.

Step 3: Fix the issue

  • Bad list: Re-verify, remove all invalid and risky addresses, reduce volume
  • Content triggered: Remove spam words, links, images; simplify formatting
  • Blacklisted: Submit delisting request, fix the cause, wait for delisting
  • Volume spike: Return to previous sending levels and ramp up gradually

Step 4: Rebuild reputation

After fixing the issue, treat the affected inbox or domain like a new one. Increase warm-up activity, reduce cold email volume to 10–15 per day, and gradually ramp back up over 1–2 weeks while monitoring metrics closely.

Step 5: Decide whether to recover or replace

Some domains recover quickly with reduced volume and time. Others are damaged too severely. If a domain still shows "Bad" reputation in Google Postmaster Tools after 3–4 weeks of remediation, it is more efficient to retire the domain and bring in a new, pre-warmed replacement.

Watch out

Reputation recovery is not instant. Even after fixing the root cause and completing delisting, it can take 1–4 weeks for email providers to update their internal reputation scores. Do not rush back to full volume during this period.

Building a monitoring routine

The best protection against deliverability crises is a simple, consistent monitoring routine. Here is what we recommend:

Daily (2 minutes):

  • Glance at bounce rates and open rates per inbox in your outreach platform
  • Check for any spam complaint notifications

Weekly (10 minutes):

  • Review Google Postmaster Tools for each sending domain
  • Run MXToolbox blacklist check on all domains
  • Send a test email from each active inbox to verify inbox placement
  • Check warm-up tool reports for inbox placement trends

Monthly (30 minutes):

  • Full audit of all domain and inbox health (the spreadsheet mentioned in Chapter 4)
  • Re-verify DNS records for all domains (records can accidentally be deleted during DNS changes)
  • Review and update domain rotation schedule
  • Assess whether any domains need retirement or replacement

This routine takes less than an hour per week and prevents the vast majority of deliverability crises. It is the difference between proactive management and reactive firefighting.

With deliverability covered — authentication, domain setup, warm-up, rotation, spam triggers, and monitoring — you have the technical foundation in place. Your emails can reach the inbox. The next module focuses on what might be the most impactful variable: finding and targeting the right people to email in the first place.