Definition
IP warmup is the deliberate, gradual ramp of email volume from a new or previously-inactive sending IP. The goal is to give mailbox providers — Gmail, Outlook, Yahoo, and the rest — time to observe how the IP behaves, what kind of engagement its mail gets, and what complaint rates it produces. A successful warmup ends with an IP that has positive reputation. Skip it, and the IP either gets throttled, rate-limited, or marked as spam by default. Domain warmup is the same concept applied to a sending domain rather than an IP.
Why mailbox providers care about new IPs
Spammers do not warm up. They acquire an IP, blast millions of messages, get blacklisted, move to the next IP, repeat. Mailbox providers know this pattern well, and the simplest defence is treating any new IP that suddenly starts pushing high volume as suspect. Reputation systems do not score IPs based on intent. They score on observed behaviour over time.
When you start sending from a fresh IP, you have no reputation. Gmail will accept your mail and decide what to do with it based on the only signals available: engagement of the recipients, complaint rates, bounce rates. Send 100K cold messages on day one and you will see high bounce + low engagement + some complaints. Gmail concludes you are spam-shaped and acts accordingly. That impression takes weeks to undo, sometimes longer.
Warmup is the opposite pattern. Day one you send to a small, highly-engaged audience. Engagement is high. Complaints are zero. Mailbox providers learn the IP as a quality sender. Day three you send a little more, still to engaged recipients. Day seven you double again. By week four you are sending real volume with established positive reputation. The whole thing is just behaviour-shaping for the spam-detection system.
A realistic warmup schedule
Industry baseline schedules vary slightly but converge on roughly this shape. Week 1: 1,000 to 2,000 sends per day, to the most engaged tier of your list. Week 2: double to 4,000-8,000 if signals stay clean. Week 3: 15,000-25,000. Week 4: 50,000-75,000. Past week 4 you can scale to whatever your normal volume is, with the IP now considered "warm." If signals deteriorate at any point — bounces above 3%, complaints above 0.1%, blocks from specific ISPs — back off to the previous level and stay there until things normalise. The ramp is gradient-aware, not calendar-aware.
Day-by-day distribution matters as much as totals. Sending 2,000 in one burst at 9 AM is different from sending 200/hour from 9 AM to 7 PM. The latter looks much more like normal human-paced traffic and is what mailbox providers expect from legitimate senders.
Our Anomaly Cortex automates this. Cortex reads real-time ISP signals every 30 seconds and adjusts your sending rate up or down based on what mailbox providers are telling it. The schedule above is what you would build manually. Cortex does it without the human-pacing tax.
When do you need to warm up?
Three situations always require warmup. First, brand-new dedicated IPs that have never sent any production mail. Reputation starts at neutral and needs building. Second, IPs that have been dormant for 30+ days. ISP reputation profiles decay; an IP that was warm three months ago is essentially cold again. Third, IPs migrating from one ESP to another, even if the IP is technically the same one — the sending domain changes, the SPF includes change, and the receiving servers see this as a fresh sending profile.
Two situations do not technically require warmup but benefit from it. Switching from a shared IP pool to a dedicated IP — same domain, new IP — the new IP needs its own ramp. Bringing a previously-quarantined IP back into rotation after a reputation incident — even more conservative ramp than a fresh IP, because some negative signal already exists.
Common warmup mistakes
Sending too much, too fast. The most common one. A founder gets a new ESP, has a 50K list ready to go, and sends to all 50K on day one. Day two they wonder why open rates are at 8%. The list was fine. The reputation was the bottleneck.
Warming with cold lists. Warmup needs engaged recipients to build positive reputation. If your initial sends go to a bought list or a list that has not been emailed in 18 months, the engagement signal is weak and the warmup actually hurts you. Use the most-engaged 5-10% of your audience for week 1. Save the unengaged tier for week 6+ once the IP has positive momentum.
Treating warmup as a one-time event. New IPs warm. Then they cool back down if they sit idle. If your sending volume drops to zero for a month — common for businesses with seasonal cadence — re-warming is necessary before the next burst. The ISPs decay your profile over time. You have to keep the signal alive.
Ignoring per-ISP signals. Your IP might be warm for Gmail and cold for Outlook simultaneously. The reputation systems are separate. If you see a sudden Outlook-specific block rate spike during warmup, slow down the Outlook portion of your sends specifically, not the overall volume. The domain temperature article covers per-ISP nuance in more depth.
Related reading
- Domain temperature and warmup — long-form on how warmup interacts with reputation scoring.
- Email Deliverability — broader context where warmup sits as one of five core practices.
- Anomaly Cortex — how sendmsg.io's reputation engine automates the warmup ramp based on live ISP signals.
- Run a free deliverability check — verifies your authentication is right before you start warmup.