IP reputation is a trustworthiness score that ISPs assign to your sending IP address. High score means inbox. A low score means spam folder — or outright blocking.
The score reflects your sending history:
- Bounce rates
- Spam trap hits
- Complaint rates
- Blacklist presence
- Authentication status
Every email you send either builds or erodes this reputation. Unlike domain reputation (which follows your brand), IP reputation is tied to specific server infrastructure. Switch IPs and you start over — good or bad.
What is IP reputation?
IP reputation works like a credit score for email. Banks examine your payment history to gauge borrower risk. ISPs examine your IP’s behavioral history to gauge sender risk.
The score typically ranges from 0 to 100, with higher numbers indicating greater trustworthiness.
| Tier | Score range | What it means |
| Good/Great | 80-100 | High inbox placement, trusted sender |
| Neutral | 70-79 | Inconsistent results, insufficient track record |
| Poor/Bad | Below 70 | Spam folder or blocked, blacklist risk |
| High-risk | Very low | Associated with malware/phishing, often blocked entirely |
A new IP starts with no history — neutral territory. Reputation builds (or degrades) based on what you send and how recipients respond. One malware infection can tank a score that took months to establish.
How do ISPs calculate IP reputation?
ISPs don’t publish exact formulas, but the signals they track are well understood. Some factors build trust while others destroy it.
Positive signals
The positive signals include:
| Factor | How it helps |
| Consistent sending volume | Predictable patterns signal legitimacy |
| High engagement | Opens and clicks prove recipients want the mail |
| Low complaint rate | Few “mark as spam” reports |
| Low bounce rate | Clean list with valid addresses |
| Authentication (SPF/DKIM/DMARC) | Proves sender identity |
| IP age | Older IPs with a clean history are viewed as stable |
Negative signals
The negative signals include:
| Factor | How it hurts |
| Spam complaints | Recipients marking messages as junk |
| High bounce rates | Sending to invalid addresses |
| Spam trap hits | Sending to honeypot addresses |
| Blacklist presence | Listed on Spamhaus, Barracuda, etc. |
| Volume spikes | Sudden increases trigger fraud detection |
| Malware/botnet activity | Compromised server sending malicious traffic |
| Bad IP neighborhood | Other IPs on the same subnet are behaving badly |
The neighborhood factor catches many senders off guard. On a shared IP, other senders’ behavior affects your reputation too (even if you’re doing everything right). One reason dedicated IPs appeal to high-volume senders — full control over the score.
How does IP reputation differ from domain reputation?
Both matter, but they measure different things. A clean IP can’t save a domain with a phishing history. A trusted domain won’t rescue mail from a blacklisted IP.
| Aspect | IP reputation | Domain reputation |
| What it measures | Trustworthiness of the sending server | Trustworthiness of brand/domain |
| Scope | Specific IP address | All mail from the domain (any IP) |
| Who controls it | Whoever manages the server | Domain owner |
| Shared IP impact | Affected by other senders | Not affected by other senders |
| Portability | Stays with IP (can’t migrate) | Follows the domain to the new infrastructure |
| Recovery speed | Can improve in weeks | Often takes longer (brand trust) |
The relationship matters for deliverability decisions. ISPs evaluate both signals before routing your email.
Think of IP reputation as a delivery truck’s driving record, and domain reputation as a shipping company’s business record. A perfectly driven truck still gets stopped if it carries cargo from a known fraudster.
How do you check your IP reputation?
Monitoring is essential — you can’t fix what you can’t see. Several free tools provide visibility into how ISPs view your sending IP.
Provider tools
Provider tools reveal how that specific mailbox provider views your IP. Gmail’s assessment may differ from Outlook’s — worth checking both if you send significant volume to either.
| Tool | Provider | What it shows |
| Google Postmaster Tools | Gmail-specific reputation tiers, spam rates, and authentication | |
| Microsoft SNDS | Microsoft | Outlook/Hotmail reputation, junk mail data |
Google Postmaster Tools and Microsoft SNDS each deserve a dedicated setup.
Third-party scoring
Sender Score provides a single 0-100 number aggregating behavior across the internet. Useful benchmark, though individual ISPs may see things differently.
| Tool | What it provides |
| Email Deliverability Test | Reputation check, inbox placement, blacklist checks, and other important metrics (for free). |
| Sender Score (Validity) | 0-100 score based on sending behavior |
| Talos Intelligence (Cisco) | Reputation lookup, threat categorization |
| Barracuda Central | Reputation status, blacklist check |
| MXToolbox | Blacklist monitoring, DNS diagnostics |
Blacklist checks
Getting listed on Spamhaus is serious — many ISPs reject mail outright from listed IPs. Regular monitoring catches listings before they crater deliverability.
| Blacklist | Focus |
| Spamhaus | Major ISP-level blocklist |
| Barracuda | Enterprise spam filtering |
| SORBS | Spam and open relay sources |
| SpamCop | User-reported spam sources |
The blacklist removal process varies by list, but all require identifying and fixing the root cause first.
What damages IP reputation?
Reputation damage usually traces to predictable sources. Understanding them helps prevent the slide before it starts.
Complaint spikes
Recipients clicking “mark as spam” send a direct signal to ISPs. Even a small percentage (above 0.1%) triggers filters. Gmail draws the line at 0.3%.
Common causes:
- Mailing too frequently
- Emailing purchased lists
- Content that looks spammy
- Sending to people who didn’t opt in
Spam complaints carry more weight than almost any other negative signal.
Bounce rates
High hard bounce rates (invalid addresses) suggest poor list hygiene. ISPs interpret bounces as a sign that you might be sending to scraped or purchased lists. Penalties follow.
Spam trap hits
Spam traps are addresses that should never receive legitimate email. Hitting them proves you’re sending to unverified addresses — either purchased lists or ancient databases that haven’t been cleaned.
Compromised servers
Malware can turn your server into a “zombie” and send spam without your knowledge. IP reputation tanks while you’re unaware (a painful discovery when deliverability suddenly collapses). Regular security audits matter more than most senders realize.
Volume spikes
Sudden jumps in sending volume trigger fraud detection. ISPs expect gradual, predictable patterns. Doubling volume overnight looks like a compromised account or spam operation — even if you’re just launching a big campaign.
How do you improve IP reputation?
Recovery isn’t instant. Rebuilding requires consistent clean signals over time (sometimes weeks, sometimes months).
Core strategies
Here are core strategies to build your IP reputation:
| Strategy | What to do |
| IP warming | Gradually increase volume on new/cold IPs |
| List hygiene | Remove invalid, inactive, and unengaged addresses |
| Authentication | Implement SPF, DKIM, and DMARC |
| Monitor complaints | Keep spam complaint rate below 0.1% |
| Separate servers | Marketing mail on a different IP than transactional |
| Malware scanning | Check for compromised servers |
| Blacklist monitoring | Catch listings early, request delisting |
| Consistent volume | Avoid sudden spikes |
Recovery timeline
Recovery gets harder with each offense. First-time blacklisting might clear in weeks. Repeated listings make ISPs skeptical of rehabilitation claims.
| Damage severity | Typical recovery |
| Minor dip (complaint spike) | 1-2 weeks of clean sending |
| Moderate (blacklisted once) | 2-4 weeks after delisting |
| Severe (malware/botnet) | 1-3 months of rehabilitation |
| Repeated offenses | Progressively harder each time |
IP warming
New IPs have no reputation — you have to build it gradually. Sending 50,000 emails on day one from a fresh IP almost guarantees placement in spam folders. Email warmup helps build the gradual volume ramp that establishes trust.
Authentication foundation
SPF, DKIM, and DMARC don’t guarantee a good reputation, but missing authentication almost guarantees a poor reputation. ISPs expect proper setup as a baseline.
What happens when reputation drops?
The business consequences extend beyond undelivered emails.
| Impact area | Consequence |
| Email deliverability | Messages land in spam or blocked |
| Marketing ROI | Campaigns fail to reach recipients |
| Transactional email | Order confirmations and password resets fail |
| Brand credibility | Recipients perceive the sender as a spammer |
| Employee productivity | Outbound communication fails |
The worst part is that you often don’t know it’s happening. Emails silently route to spam without bounce notifications. Marketing campaigns “fail” with no obvious explanation.
Monitoring tools are the only early warning system — by the time customers complain about missing emails, the damage is done.
Frequently asked questions
Here are some commonly asked questions about IP reputation:
Yes, but it takes time. First, identify and fix the root cause (malware, bad list, complaint spike). Then request delisting from the specific blacklist. Finally, send clean traffic consistently for weeks while the reputation rebuilds. Each subsequent listing makes recovery harder.
Technically, yes — new IP means new reputation (neutral starting point). But ISPs aren’t naive. Domain reputation carries forward, and aggressive volume from a new IP raises red flags. Changing IPs to escape a bad reputation usually backfires; fixing the underlying behavior is the only sustainable path.
Weekly for steady senders. Daily during campaigns or after infrastructure changes. Immediately, if deliverability suddenly drops. Set up alerts in tools like MXToolbox to catch blacklistings without manual checking.
