Email Deliverability Monitoring Tools

Why Monitoring Matters

Email deliverability is not a set-and-forget configuration. ISPs continuously update their spam filters, your sender reputation fluctuates with every send, and blacklists can flag your IP without warning. Without monitoring, you will not know about problems until users complain — and most never do, they just stop engaging.

Free ISP Tools

Google Postmaster Tools

Google Postmaster Tools provides direct insight into how Gmail views your domain and IPs:

• Domain reputation — High, Medium, Low, or Bad. Directly correlates to inbox placement.
• IP reputation — Per-IP reputation for each sending IP.
• Spam rate — Percentage of emails that users marked as spam.
• Authentication — SPF, DKIM, and DMARC pass rates.
• Encryption — TLS usage percentage.

Setup: Verify your domain at https://postmaster.google.com/, add a DNS TXT record, and data appears within 24-48 hours.

Microsoft SNDS

Microsoft Smart Network Data Services shows how Outlook.com views your sending IPs:

• IP status — Green (good), Yellow (mixed), Red (bad).
• Spam trap hits — Number of messages sent to known spam traps.
• Complaint rate — Per-IP complaint percentages.

Register at https://sendersupport.olc.protection.outlook.com/snds/.

Yahoo Complaint Feedback Loop

Yahoo provides a Complaint Feedback Loop (CFL) that notifies you when users mark your email as spam. Register at https://io.help.yahoo.com/contact/index to receive feedback loop notifications for your sending domain.

Blacklist Monitoring

MXToolbox

MXToolbox checks your IP against 100+ blacklists simultaneously:

# Check your IP against major blacklists
# Using MXToolbox (web-based)
# https://mxtoolbox.com/blacklists.aspx

# Using command line (check Spamhaus)
dig +short 100.51.198.203.zen.spamhaus.org
# If listed: returns 127.0.0.x (where x indicates the list)
# If clean: returns NXDOMAIN (no result)

# Check Barracuda
dig +short 100.51.198.203.b.barracudacentral.org

# Check SpamCop
dig +short 100.51.198.203.bl.spamcop.net

Set up free monitoring to receive email alerts when your IP appears on any major blacklist. The free tier monitors one IP with daily checks.

Automated Monitoring Script

# Check for unauthorized relay
grep "relay=" /var/log/mail.log | grep -v "relay=127.0.0.1"

# Check outbound email volume (unusually high = compromised)
grep "status=sent" /var/log/mail.log | wc -l

# Verify mail server is not an open relay
telnet your-server.com 25
EHLO test
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
# Should respond with "Relay access denied"

Run this as a daily cron job and alert on any listings.

Seed Testing

Seed testing measures inbox placement — whether your email lands in the inbox, spam, or is missing entirely:

The process: Send your actual email campaign to a list of seed addresses maintained by the tool. The tool checks where the email landed (inbox, spam, missing) at each ISP.

Authentication Monitoring

Verify your SPF, DKIM, and DMARC are working correctly:

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DMARC Aggregate Reports

Receiving servers send daily DMARC aggregate reports to the address in your DMARC record (rua=mailto:). These XML reports show:

• Which IPs sent email using your domain
• SPF and DKIM pass/fail rates per IP
• DMARC alignment results

Use a DMARC report analyzer to visualize this data. It will reveal unauthorized senders, misconfigured SPF includes, and DKIM signing failures.

Building a Monitoring Dashboard

Combine these data sources into a single dashboard:

Review this dashboard weekly. Investigate any metric that crosses its threshold immediately — deliverability problems compound quickly if ignored.