In 2019, a well-funded startup launched their product with a massive email campaign. They'd built a list of 200,000 signups during their beta period. On launch day, they sent everyone an announcement email. Within hours, Gmail was rejecting their messages entirely. Their domain had been flagged as a spam source.
The emails weren't spam—they were legitimate announcements to people who'd signed up. But the startup had made a critical mistake: they'd never sent email from that domain before. To Gmail's algorithms, a brand-new domain suddenly sending 200,000 emails looked exactly like a spammer who'd just registered a throwaway domain.
Email reputation is the invisible score that determines whether your messages reach their destination. Understanding how it works is essential for anyone sending email at scale.
What reputation actually means
Email reputation isn't a single number you can look up. It's a collection of signals that receiving servers use to decide how much to trust your email. Different providers calculate it differently, weight factors differently, and make different decisions based on it.
Think of it like a credit score, but more opaque. You can't see the exact number. You can't dispute specific items. You can only observe the effects—whether your emails are delivered, filtered, or rejected—and infer your standing from there.
Reputation exists at multiple levels. Your IP address has a reputation based on what's been sent from it historically. Your domain has a reputation based on emails claiming to be from that domain. Even specific email addresses can develop reputations. All of these factor into delivery decisions.
The major email providers—Gmail, Microsoft, Yahoo—each maintain their own reputation systems. Your reputation with Gmail might be excellent while your reputation with Microsoft is poor. This is why deliverability can vary dramatically across providers.
The signals that matter
ISPs look at dozens of signals when calculating reputation. Some are about your technical setup; others are about recipient behavior.
Spam complaints are the most damaging signal. When someone clicks "Report Spam" on your email, that's a direct statement that they didn't want it. Even a small complaint rate—0.1% is often cited as a threshold—can tank your reputation. This is why easy unsubscription matters: people who can't find the unsubscribe link hit the spam button instead.
Bounce rates matter too. If you're sending to addresses that don't exist, you're either using a purchased list (bad) or not maintaining your list hygiene (also bad). High bounce rates suggest you don't have a legitimate relationship with your recipients.
Engagement signals are increasingly important. Gmail in particular watches whether recipients open your emails, click links, reply, or move messages out of spam. High engagement suggests wanted email; low engagement suggests the opposite. This creates a feedback loop: emails that get good engagement build reputation, which improves inbox placement, which enables more engagement.
Spam trap hits are particularly damaging. Spam traps are email addresses that shouldn't receive any legitimate email—either because they were never valid or because they've been abandoned for years. Sending to them proves you're either scraping addresses or not cleaning your list. One spam trap hit can outweigh thousands of successful deliveries.
Authentication matters as a baseline. SPF, DKIM, and DMARC passing doesn't guarantee good reputation, but failing them guarantees bad reputation. Authentication is table stakes; you need it to even be considered for the inbox.
IP reputation vs domain reputation
Historically, IP reputation was everything. Spammers would burn through IP addresses, sending until they got blocked, then moving to new IPs. Blocking by IP was effective because legitimate senders had stable infrastructure.
This has shifted. Cloud computing means IP addresses are shared and recycled constantly. A spammer might use the same cloud provider as your legitimate business. Blocking by IP would cause too much collateral damage.
Domain reputation has become more important as a result. Your domain is stable—you're not going to abandon it like a spammer abandons IPs. Building reputation on your domain creates a persistent identity that follows you regardless of infrastructure changes.
This is why using your own domain for email matters. If you send from a shared domain (like many free email marketing tools offer), you inherit the reputation of everyone else using that domain. One bad actor can poison the well for everyone.
For transactional email, using a subdomain (like mail.yourdomain.com) can isolate reputation. If your marketing emails cause problems, they won't drag down your transactional email deliverability, and vice versa.
How reputation is built
Building email reputation is like building credit history: it takes time, consistency, and good behavior. There are no shortcuts.
Start slowly. If you're sending from a new domain or IP, begin with small volumes to your most engaged recipients. These are people most likely to open, click, and not complain. Their positive engagement signals establish your initial reputation.
Increase volume gradually. The "warming" process typically takes 4-8 weeks. You might start with 100 emails per day, increase to 500, then 1,000, then 5,000, scaling up as your reputation solidifies. Jumping straight to high volume looks like spam behavior.
Maintain consistency. Sending patterns matter. If you normally send 10,000 emails per day and suddenly send 100,000, that spike looks suspicious. Consistent, predictable sending patterns build trust.
Monitor feedback loops. Major ISPs offer feedback loop programs that notify you when recipients mark your email as spam. Sign up for these. They're early warning systems that let you identify problems before they tank your reputation.
Keep your list clean. Remove bounced addresses immediately. Remove unengaged subscribers periodically. Never, ever buy or rent email lists. The short-term volume isn't worth the long-term reputation damage.
When reputation goes wrong
Reputation damage can happen suddenly. A compromised account sending spam, a bad list purchase, a campaign that triggers mass complaints—any of these can crater your reputation in hours.
The symptoms are obvious: delivery rates plummet, emails start landing in spam, or you see outright rejections. The cause isn't always obvious. You might not know about the compromised account or realize that the "partnership list" your marketing team acquired was actually scraped data.
Recovery is slow. You can't just apologize and have your reputation restored. You need to fix the underlying problem, then rebuild trust through consistent good behavior. This typically takes weeks to months, depending on how severe the damage was.
During recovery, reduce volume dramatically. Send only to your most engaged recipients. Focus on generating positive signals—opens, clicks, replies—rather than volume. Gradually expand as metrics improve.
Some reputation damage is permanent, or close to it. If your domain becomes associated with serious abuse, you might need to start fresh with a new domain. This is a last resort, but sometimes it's faster than trying to rehabilitate a thoroughly poisoned reputation.
Monitoring your reputation
You can't manage what you can't measure. Several tools help you understand your reputation standing.
Google Postmaster Tools is essential if you send to Gmail users (and you probably do). It shows your domain and IP reputation, spam rates, authentication success rates, and delivery errors. The data is delayed by a day or two, but it's authoritative—it's Google telling you what they think of you.
Microsoft SNDS (Smart Network Data Services) provides similar insights for Outlook and Hotmail. The interface is less polished than Google's, but the data is valuable.
Third-party tools like Sender Score (from Validity) aggregate reputation data across multiple sources. They're useful for a quick check but less authoritative than data from the ISPs themselves.
Your own metrics matter too. Track delivery rates, open rates, click rates, bounce rates, and complaint rates over time. Sudden changes in any of these can indicate reputation problems before they become severe.
Frequently asked questions
How long does it take to build email reputation?
For a new domain or IP, expect 4-8 weeks of gradual warming before you can send at full volume with good deliverability. Reputation continues to develop over months and years based on ongoing sending behavior.
Can I buy a domain with existing good reputation?
Technically yes, but it's risky. The previous owner's reputation transfers with the domain, but so does any negative history. And if you change sending patterns dramatically, ISPs may reevaluate. Building your own reputation is safer.
Does sending more email improve reputation?
Not directly. Volume alone doesn't build reputation—engagement does. Sending more email to unengaged recipients will hurt your reputation. Sending to engaged recipients who open and click builds it.
Why is my reputation different at Gmail vs Microsoft?
Each provider maintains independent reputation systems with different algorithms and thresholds. Your sending patterns might trigger different responses. A campaign that Gmail likes might not resonate with Microsoft's filters, or vice versa.