IP and Domain Reputation: The Hidden Factor in Email Deliverability

Your email infrastructure can be technically perfect-SPF, DKIM, DMARC all passing-and emails still land in spam or get rejected. The missing piece is reputation: the trust score that mailbox providers assign to your sending IPs and domains based on historical behavior.

Reputation is earned over time and destroyed in moments. A compromised account sending spam, a misconfigured application blasting emails, or even aggressive marketing practices can tank your reputation overnight. This guide covers how reputation works, how to monitor it, and how to recover when things go wrong.

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How Reputation Works

Mailbox providers (Gmail, Microsoft, Yahoo) maintain reputation scores for every IP address and domain that sends them email. These scores determine whether your messages reach the inbox, land in spam, or get rejected outright.

Positive Signals	Negative Signals
✓ Low bounce rates	✗ High bounce rates
✓ Low spam complaints	✗ Spam complaints
✓ High engagement (opens/clicks)	✗ Spam trap hits
✓ Consistent sending volume	✗ Sudden volume spikes
✓ Clean list hygiene	✗ Blacklist appearances
✓ Authentication (SPF/DKIM/DMARC)	✗ Authentication failures
✓ Proper unsubscribe handling	✗ High unsubscribe rates

Score Range	Rating	Impact
90-100	EXCELLENT	Priority inbox placement
70-89	GOOD	Normal inbox delivery
50-69	NEUTRAL	Some spam folder placement
25-49	POOR	Significant spam filtering
0-24	BAD	Rejections, blocking

IP Reputation vs Domain Reputation

Both matter, but they work differently:

Factor	IP Reputation	Domain Reputation
What it tracks	Sending server IP address	Your domain (From, DKIM)
Persistence	Tied to IP; changes if you move	Follows your domain everywhere
Shared vs Dedicated	Often shared on ESP platforms	Always specific to your domain
Recovery time	Days to weeks	Weeks to months
Primary use	Connection-level filtering	Content-level filtering

IP reputation matters most for connection-level decisions: will the receiving server even accept the connection? Low IP reputation results in connection timeouts, rate limiting, or outright blocks.

Domain reputation influences content-level decisions: once accepted, does the message go to inbox or spam? Domain reputation increasingly dominates because senders can change IPs but not domains.

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Monitoring Your Reputation

Don't wait for delivery problems to discover reputation issues. Proactive monitoring catches problems early.

Google Postmaster Tools

Google Postmaster Tools provides direct visibility into how Gmail views your domain:

Metric	Status
Domain Reputation	HIGH
IP Reputation	MEDIUM
Spam Rate	0.3%
SPF Success	99.8%
DKIM Success	99.9%
DMARC Success	99.7%
TLS Encryption	100%

Setup:

1. Go to postmaster.google.com
2. Add and verify your domain (DNS TXT record)
3. Monitor daily once you have sufficient volume (100+ emails/day to Gmail)

Microsoft SNDS

Microsoft Smart Network Data Services provides similar insights for Outlook/Hotmail:

1. Visit sendersupport.olc.protection.outlook.com/snds
2. Request access for your IP ranges
3. Monitor spam complaints and trap hits

Third-Party Reputation Services

Service	What It Measures	Cost
Sender Score (Validity)	IP reputation 0-100	Free
Talos Intelligence (Cisco)	IP/domain reputation	Free
Barracuda Central	IP reputation	Free
MXToolbox	Blacklist monitoring	Freemium
250ok / Validity	Comprehensive reputation	Paid

Sender Score (senderscore.org) provides a 0-100 score for any IP address. Scores above 80 generally indicate good reputation.

# Quick reputation check
curl -s "https://www.senderscore.org/lookup.php?lookup=YOUR_IP"

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Blacklists: Detection and Removal

Blacklists (DNSBLs) are real-time databases of IPs known to send spam. Major providers check these lists when receiving email.

Major Blacklists

Blacklist	Impact	Removal
Spamhaus (SBL, XBL, PBL)	Severe	Manual request
Barracuda	Significant	Automatic after 12hrs
SpamCop	Moderate	Automatic decay
SORBS	Moderate	Manual request
CBL (Composite)	Moderate	Automatic after cleanup
URIBL / SURBL	Domain blacklists	Manual request

Checking Blacklist Status

# Check if IP is on Spamhaus
dig +short 4.3.2.1.zen.spamhaus.org

# Response of 127.0.0.x means listed
# No response means clean

# Multi-blacklist check
# Use MXToolbox: mxtoolbox.com/blacklists.aspx

Blacklist Removal Process

1. Identify the Cause

   • Review mail logs for unusual activity
   • Check for compromised accounts
   • Audit application sending behavior
   • Identify spam trap hits

2. Fix the Underlying Issue

   • Secure compromised accounts
   • Fix application bugs
   • Clean mailing lists
   • Implement rate limiting

3. Request Removal

   • Spamhaus: spamhaus.org/lookup
   • Barracuda: barracudacentral.org/lookups
   • SpamCop: spamcop.net/bl.shtml

4. Monitor for Re-listing

   • Set up automated blacklist monitoring
   • Alert on new listings
   • Investigate immediately if re-listed

⚠️ WARNING: Requesting removal without fixing the cause results in immediate re-listing

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Common Reputation Killers

Spam Traps

Spam traps are email addresses that should never receive legitimate email. Hitting them signals poor list hygiene.

Trap Type	Description	How to Avoid
Pristine	Addresses that never opted in	Never purchase lists
Recycled	Abandoned addresses repurposed	Regular list cleaning
Typo	Common typo domains (gmial.com)	Email validation at signup

Purchased Lists

Purchasing email lists is the fastest path to reputation destruction. These lists contain:

• Spam traps planted to catch purchasers
• Outdated addresses that bounce
• People who never consented

One campaign to a purchased list can blacklist your IP and damage your domain reputation for months.

Volume Spikes

Mailbox providers notice sudden volume changes. A domain that sends 100 emails/day suddenly sending 10,000 triggers spam filtering.

Pattern	Behavior	Result
Suspicious	Sudden volume spike from normal levels	Triggers spam filtering
Healthy	Gradual volume increase over time	Builds trust with providers

Complaint Rates

When recipients click "Report Spam," mailbox providers track this. High complaint rates destroy reputation:

Complaint Rate	Impact
< 0.1%	Healthy
0.1% - 0.3%	Warning zone
0.3% - 0.5%	Reputation damage
> 0.5%	Severe filtering/blocking

Gmail considers > 0.3% problematic. Even legitimate senders exceed this if they mail disengaged subscribers.

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Building and Protecting Reputation

IP Warming

New IPs have no reputation-they're unknown entities. Warming gradually builds positive history:

Day	Volume/Day	Target Recipients
1-2	50	Most engaged subscribers only
3-4	100	High engagement
5-7	250	High engagement
8-10	500	High + medium engagement
11-14	1,000	Medium engagement
15-21	2,500	Medium engagement
22-28	5,000	Broader audience
29-35	10,000	Full list minus inactives
36+	Scale up 25%/week	As reputation builds

⚠️ IMPORTANT:

• Only send to opt-in recipients during warmup
• Monitor bounces and complaints closely
• Slow down if you see deliverability issues
• Warmup separately for each major provider

List Hygiene

Clean lists regularly to maintain reputation:

1. Remove hard bounces immediately: Invalid addresses hurt reputation
2. Suppress complaints: Never email anyone who complained
3. Sunset inactive subscribers: Users who don't engage for 6-12 months hurt metrics
4. Validate at signup: Block typos and fake addresses

Authentication Hygiene

Authentication failures damage domain reputation:

# Monitor DMARC reports for authentication issues
# Example aggregate report summary:

Source IP        SPF    DKIM   DMARC   Count
---------------------------------------------
192.0.2.1        pass   pass   pass    15,234
192.0.2.2        fail   pass   pass     1,203
198.51.100.5     fail   fail   fail       847  ← Unknown sender!
203.0.113.99     pass   fail   fail       423  ← DKIM broken

Investigate and fix all authentication failures. The fail/fail/fail entries are either spoofing attempts or misconfigured legitimate senders-both need attention.

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Reputation Recovery

When reputation tanks, recovery takes time and discipline.

Immediate Actions

1. Stop all non-essential sending: Reduce volume to minimum
2. Identify the cause: Logs, DMARC reports, blacklist notices
3. Fix the root cause: Before any recovery can begin
4. Clean your list: Remove bounces, complaints, inactives

Recovery Timeline

Reputation Level	Expected Recovery Time
Minor dip (70-80)	1-2 weeks
Moderate damage (50-69)	2-4 weeks
Severe damage (25-49)	1-3 months
Blacklisted (< 25)	3-6 months

Recovery Strategy

Phase 1: Stabilization (Week 1-2)

• Reduce sending volume by 80%
• Send only to most engaged users
• Monitor all metrics closely
• Request blacklist removals (if applicable)

Phase 2: Cleanup (Week 3-4)

• Audit and clean entire list
• Fix all authentication issues
• Review and fix sending applications
• Implement better monitoring

Phase 3: Rebuilding (Week 5+)

• Gradually increase volume (10-20%/week)
• Continue monitoring metrics
• Adjust based on deliverability
• Full recovery when metrics stabilize

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Reputation Monitoring Checklist

Set up ongoing monitoring before problems occur:

Check	Frequency	Tool
Blacklist status	Daily	MXToolbox, automated
Google Postmaster	Daily	Postmaster Tools
Microsoft SNDS	Daily	SNDS Dashboard
Sender Score	Weekly	senderscore.org
Bounce rates	Per campaign	ESP dashboard
Complaint rates	Per campaign	ESP + feedback loops
DMARC reports	Weekly	DMARC analyzer

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Next Steps

Reputation management is ongoing. Build monitoring before you have problems, maintain clean lists, authenticate everything, and respond quickly to issues. Professional email deliverability services can help organizations maintain strong sender reputation and avoid blacklisting.

We help organizations monitor and recover email reputation through our email security services. Request an assessment to evaluate your current sending reputation and identify risks.

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Dealing with deliverability problems or blacklisting? We've recovered reputation for organizations facing severe blacklisting and helped rebuild sending infrastructure from scratch. Contact us for immediate assistance.