You've just set up a new email infrastructure. Fresh IP addresses, clean domain, everything configured perfectly. You're ready to send. So you load up your 500,000-subscriber list and hit send.
Within hours, you're blocked by Gmail. Outlook is rejecting your connections. Your emails are bouncing with cryptic error messages about 'suspicious activity.' What went wrong?
You skipped the warmup.
Email providers are deeply suspicious of new senders. A brand-new IP address with no history suddenly sending thousands of emails looks exactly like a spammer who just spun up fresh infrastructure. The only way to prove you're legitimate is to start small and build trust gradually. This process is called IP warmup, and it's non-negotiable for anyone sending significant email volume.
Why warmup matters
Email providers maintain reputation scores for every IP address that sends them email. A new IP has no reputation—it's a blank slate. And in the world of email, no reputation is treated almost as badly as bad reputation.
Think about it from the provider's perspective. They see millions of emails per day. Most spam comes from IPs with no established history—spammers constantly rotate through new infrastructure to evade blocks. A new IP sending high volume is statistically likely to be spam.
Warmup is how you demonstrate that you're different. By starting with low volume and gradually increasing, you give providers time to observe your sending patterns. They see that recipients engage with your emails, that you handle bounces properly, that you're not generating complaints. Each day of good behavior builds trust.
The alternative—sending high volume immediately—triggers automatic defenses. You'll hit rate limits, get temporarily blocked, or have your emails routed to spam. Even if you're completely legitimate, you'll look like a spammer, and recovering from that first impression is harder than building reputation correctly from the start.
The warmup process
A typical warmup schedule spans 4-8 weeks, starting with very low volume and roughly doubling every few days. The exact numbers depend on your total target volume and the providers you're sending to.
Day 1 might be 50-100 emails. Day 3, maybe 200. Day 5, 500. Day 7, 1,000. You continue this progression until you reach your normal sending volume. The key is gradual, consistent growth—no sudden spikes.
During warmup, send to your most engaged subscribers first. These are people who regularly open and click your emails. Their positive engagement signals to providers that your email is wanted, accelerating reputation building.
Avoid sending to cold or unengaged segments during warmup. If your first emails go to people who ignore them (or worse, mark them as spam), you're building negative reputation instead of positive. Save the re-engagement campaigns for after warmup is complete.
Monitor your metrics obsessively during warmup. Watch for bounces, complaints, and blocks. If you see problems, slow down or pause. It's better to extend warmup by a week than to damage your reputation by pushing too hard.
Warmup by provider
Different email providers have different thresholds and behaviors, so sophisticated warmup strategies segment by recipient domain.
Gmail is typically the most forgiving during warmup, but also the most important to get right given their market share. They provide good feedback through Postmaster Tools, so you can monitor your reputation as it builds.
Microsoft (Outlook, Hotmail, Office 365) can be more aggressive about blocking new senders. They're particularly sensitive to complaint rates. Consider warming up Microsoft domains more slowly than Gmail.
Yahoo and AOL (now part of Verizon Media) have their own quirks. They're generally in the middle—not as forgiving as Gmail, not as strict as Microsoft.
Corporate domains vary wildly. Some use aggressive filtering; others barely filter at all. You can't warm up to every corporate domain individually, but be aware that enterprise recipients might have different experiences than consumer recipients.
A common strategy is to warm up each major provider separately, starting with Gmail, then adding Microsoft, then Yahoo, then everyone else. This lets you build reputation with each provider without one provider's issues affecting others.
When warmup goes wrong
Even with careful planning, warmup doesn't always go smoothly. Knowing how to respond to problems is as important as the initial plan.
If you get temporarily blocked, stop sending to that provider immediately. Wait 24-48 hours, then resume at a lower volume. Don't try to push through blocks—you'll make things worse.
If your bounce rate spikes, check your list quality. You might have bad addresses that are triggering spam traps or generating complaints. Clean your list before continuing.
If engagement is lower than expected, reconsider your segment selection. Maybe your 'engaged' subscribers aren't as engaged as you thought, or maybe there's a content problem making people less likely to interact.
If you see spam complaints, that's a serious warning sign. Even a few complaints during warmup can derail the process. Investigate immediately—are you sending to people who didn't opt in? Is your content misleading? Is your unsubscribe process broken?
Sometimes the right response is to pause warmup entirely, fix the underlying issue, and restart. It's frustrating to lose progress, but continuing with a broken process just digs a deeper hole.
Maintaining reputation after warmup
Completing warmup isn't the end—it's the beginning. The reputation you've built needs ongoing maintenance.
Consistency matters. If you warm up to 100,000 emails per day and then only send 10,000 for a month, your reputation can decay. Providers expect consistent behavior from established senders. Sudden changes—in either direction—raise flags.
If you need to significantly increase volume after warmup, you'll need to warm up again. Going from 100,000 to 500,000 overnight will trigger the same suspicions as a new IP. Plan volume increases as mini-warmups.
Continue monitoring the same metrics you watched during warmup. Reputation can erode over time if you're not paying attention. Catch problems early, before they become crises.
If you add new IPs to your infrastructure, each one needs its own warmup. You can't transfer reputation from one IP to another. Some senders maintain a pool of warmed IPs so they have capacity for growth without starting from scratch.
Frequently asked questions
How long does warmup take?
Typically 4-8 weeks to reach full volume, depending on your target volume and how smoothly the process goes. Rushing warmup is counterproductive—it's better to take longer and build solid reputation.
Can I skip warmup if I'm using a shared IP?
Shared IPs are already warmed by other senders, so you don't need to warm them yourself. However, you're also sharing reputation with those other senders, which has its own risks.
What if I need to send urgently during warmup?
You have limited options. You could use a separate, already-warmed IP for urgent sends. Or you could accept that some emails might not be delivered optimally. Don't abandon warmup for short-term needs.
Do I need to warm up my domain too?
Domain reputation builds alongside IP reputation, but there's no separate 'domain warmup' process. As you warm up your IP with properly authenticated email, your domain reputation builds automatically.