Published April 26, 2026 · 14 min read
SPF, DKIM, DMARC, and MTA-STS configuration directly impact your sender reputation. Get an instant audit.
Free Domain Check →Every time you send an email, the receiving mail server makes a split-second decision: inbox or spam? The biggest factor in that decision is your email sender reputation — a score that ISPs build over time based on your sending behavior. Think of it as a credit score for email. A high reputation means your emails land in the inbox. A low reputation means they're filtered, blocked, or silently discarded.
This guide explains exactly how sender reputation works, how to check your current reputation, and the most effective strategies to improve it.
ISPs track two separate reputations for every email they receive:
| Factor | IP Reputation | Domain Reputation |
|---|---|---|
| What it tracks | The sending IP address | The sending domain name |
| Scope | Affects all mail from that IP | Affects all mail from that domain, on any IP |
| How to check | Sender Score, SNDS, Talos | Google Postmaster Tools |
| Resets if you change... | IP address | Domain name |
| Which matters more? | Equal for Gmail | Domain reputation is increasingly dominant at most ISPs |
Domain reputation is becoming more important than IP reputation. Modern ISPs like Gmail evaluate your domain's sending history regardless of which IP sends the email. This means switching IPs to escape a bad reputation doesn't work anymore — the domain follows you.
The percentage of recipients who click "Report Spam" or "Mark as Junk." This is the single most important reputation signal. ISPs weigh it heavily because it's a direct user signal that your email is unwanted.
| Complaint Rate | Impact |
|---|---|
| Below 0.1% | 🟢 Excellent — no impact |
| 0.1% - 0.3% | 🟡 Warning zone — ISPs start watching |
| 0.3% - 0.5% | 🟠 Problem — spam filtering increases |
| Above 0.5% | 🔴 Critical — bulk spam folder or blocking |
How to improve: Use double opt-in, set clear expectations about frequency, make unsubscribe easy (and immediate), and never send to purchased lists.
The percentage of emails that can't be delivered. Hard bounces (permanent failures — user doesn't exist) hurt your reputation more than soft bounces (temporary failures — mailbox full, server down).
| Bounce Rate | Impact |
|---|---|
| Below 2% | 🟢 Healthy |
| 2% - 5% | 🟡 Needs attention |
| 5% - 10% | 🟠 Significant — reputation damage |
| Above 10% | 🔴 Critical — likely to be blocked |
How to improve: Clean your list regularly. Remove hard bounces immediately. Suppress addresses that have bounced 2+ times in 30 days. Check if you're on blacklists →
ISPs track how recipients interact with your emails: opens, clicks, replies, forwards, and deletions-without-reading. High engagement = good reputation; low engagement = poor reputation.
SPF, DKIM, and DMARC pass rates directly affect your reputation. ISPs track what percentage of your email passes authentication:
# Check your authentication configuration curl https://korpo.pro/api/v1/check/yourdomain.com
ISPs expect consistent sending behavior. Drastic volume changes are suspicious:
Being listed on a major DNSBL (DNS-based Block List) devastates your reputation. ISPs query blacklists before accepting email, and listed IPs/domains are automatically penalized or blocked.
| Blacklist | Impact |
|---|---|
| Spamhaus SBL/DBL | 🔴 Critical — severe delivery impact |
| Barracuda | 🟠 High — major delivery impact |
| SORBS | 🟠 High — affects many receivers |
| SpamCop | 🟡 Medium — auto-delists after 24h |
| URIBL | 🟡 Medium — domain-based listing |
Check if you're on any blacklists →
Modern ISPs (especially Gmail) use AI and machine learning to evaluate email content and user-level behavior:
The single most important tool for Gmail delivery (Gmail handles 27%+ of all email). Shows your domain's reputation with Google:
Access at postmaster.google.com — you need to verify your domain ownership.
Shows your IP reputation with Outlook/Hotmail. Provides data on volume, complaint rates, and whether your IP is on Microsoft's block list.
Access at sendersupport.olc.protection.outlook.com
Provides a 0-100 reputation score for your sending IP. Scores above 90 are excellent; below 70 is concerning. Uses data from a network of spam traps and monitoring points.
Access at senderscore.org
Shows your IP/domain reputation as used by Cisco's email security products. Important because many enterprise firewalls use Talos data.
Access at talosintelligence.com
Check the DNS authentication foundation that underlies your reputation:
curl https://korpo.pro/api/v1/check/yourdomain.com
Returns a 0-100 deliverability score broken down by SPF, DKIM, DMARC, MTA-STS, and BIMI configuration.
Your DMARC aggregate reports reveal authentication pass/fail rates — a key reputation signal. Set up rua reporting to receive daily summaries.
Properly configured SPF, DKIM, and DMARC are the fastest reputation improvement you can make. Many domains have configuration errors that cause authentication failures:
New domains and IPs have zero reputation — ISPs evaluate them cautiously. A proper warmup builds positive history gradually:
Read the complete email warmup guide →
If you send more than 100,000 emails per month, consider a dedicated IP. This isolates your reputation from other senders on shared IPs:
| Factor | Shared IP | Dedicated IP |
|---|---|---|
| Reputation | Shared with other senders | Yours alone |
| Risk | Other senders' bad behavior affects you | Your behavior, your responsibility |
| Cost | Free with most ESPs | $20-50/month extra |
| Warmup needed? | No (already warmed) | Yes (2-4 weeks) |
| Best for | Under 100K emails/month | Over 100K emails/month |
DMARC enforcement (p=reject) protects your domain from spoofing, which prevents reputation damage from unauthorized senders:
# Phase 1: Monitor (2-4 weeks) _dmarc.yourdomain.com TXT "v=DMARC1;p=none;rua=mailto:dmarc@yourdomain.com" # Phase 2: Quarantine (2 weeks) _dmarc.yourdomain.com TXT "v=DMARC1;p=quarantine;rua=mailto:dmarc@yourdomain.com;pct=50" # Phase 3: Reject (ongoing) _dmarc.yourdomain.com TXT "v=DMARC1;p=reject;rua=mailto:dmarc@yourdomain.com"
Learn how to read DMARC reports →
If your reputation is already damaged, here's what to expect:
| Action | Impact Time | Effect |
|---|---|---|
| Fix DNS authentication (SPF/DKIM/DMARC) | 1-3 days | Immediate improvement in authentication pass rate |
| Stop sending to inactive contacts | 3-7 days | Lower bounces and complaints |
| Honor all unsubscribes immediately | 1-3 days | Complaint rate drops |
| Reduce sending volume | 1-2 weeks | ISPs re-evaluate cautiously |
| Get delisted from blacklists | 1-7 days | Removes hard blocks |
| Restore domain reputation | 2-6 months | ISPs evaluate 30-90 day trends |
| IP reputation recovery | 1-3 months | Depends on severity |
Reputation monitoring should be an ongoing practice, not a one-time check:
include: directives — remove unused services# Set up a weekly automated check
0 9 * * 1 curl -s https://korpo.pro/api/v1/check/yourdomain.com > /tmp/reputation-check.txt
# Check multiple domains
0 9 * * 1 curl -s -X POST https://korpo.pro/api/v1/batch \
-H "Content-Type: application/json" \
-d '{"domains":["yourdomain.com","news.yourdomain.com","mail.yourdomain.com"]}' \
> /tmp/reputation-batch.txt
Sender reputation is determined by many factors, but they're not equally weighted. Here's the priority order for impact:
Improving #1 and #2 (complaint rate and authentication) gives the fastest, most reliable reputation boost. Everything else is optimization.
Email sender reputation is a score that ISPs assign to your sending IP address and domain based on your email sending behavior. It's calculated from factors like spam complaint rates, bounce rates, engagement, and authentication configuration. A high reputation means better inbox placement; a low reputation means more spam filtering.
Use Google Postmaster Tools (for Gmail delivery), Microsoft SNDS (for Outlook), Sender Score by Validity, and Talos Intelligence by Cisco. Also check your domain's SPF, DKIM, and DMARC configuration using a tool like korpo.pro's free domain checker.
On Sender Score (0-100 scale), a score above 90 is excellent, 80-89 is good, 70-79 is fair, and below 70 is poor. For Google Postmaster Tools, a "High" domain reputation is ideal. Most ISPs consider scores above 85 as trustworthy.
Rebuilding a damaged email sender reputation typically takes 2-6 months, depending on the severity of the damage. If your domain is blacklisted, delisting takes 1-7 days, but restoring full deliverability takes longer because ISPs evaluate historical trends over 30-90 day windows.
Only temporarily, and only for IP reputation. Domain reputation follows your domain name regardless of IP. Additionally, new IPs have no reputation history, so ISPs treat them cautiously. It's almost always better to fix your existing reputation than to start over.
It can. On a shared IP, other senders' bad behavior (high complaint rates, blacklisting) affects your deliverability because the IP reputation is shared. If you send over 100,000 emails per month, a dedicated IP is worth the $20-50/month cost. Under 100,000, shared IPs at reputable ESPs are generally fine.
SPF, DKIM, DMARC, and MTA-STS configuration are the foundation of sender reputation. Get an instant audit.
Free Domain Check →