How Long Should I Warm Up a New Mailbox Before Cold Email?

The timeline

For a .info or another cheaper extension, add a couple of weeks before full volume. See .info vs .com.

Why at least two weeks

A new mailbox on a new domain has no sending history. Warmup builds one: the mailbox exchanges emails with a network of other mailboxes that open them, reply and move them out of spam. Warmup is required, and a large part of the result depends on it. Mailboxes that skip it and go straight to full cold volume do worse, and they drag down the results of the rest of your setup when mixed in.

Google's sender guidelines describe the same principle for any new sender: "Start with a low sending volume to engaged users, and slowly increase the volume over time," and "Avoid introducing sudden volume spikes if you do not have a history of sending large volumes."

What our data shows

The same warmup ran on both servers. Outlook scores stayed near 98 on each, while the Google score depended on the server the mailboxes sent from. On the degraded server, extra days of warmup did not move the score.

Domain age mattered much less. After the first month, domain age barely matters: on cold email to Google recipients, domains older than a month got 1.07% replies and newer ones 1.01%, a difference that is not statistically significant (Outreach2day data, 2026: 3.7 million sends in 7 workspaces). The first weeks of a new domain are a separate risk that our data did not measure, which is why warmup runs 14 days before the first cold email and keeps running after it. The domain age checker shows when each of your domains was registered.

What slows warmup down

• A degraded sending server. In our data this decided the result: the same mailboxes reached 85 to 90 on a healthy server and stalled at 63 to 75 on a degraded one. If Google scores stall while Outlook stays high, ask your provider about the server and its IPs; a longer warmup will not lift them.

• Warmup that isn't running. "Connected" in a sending tool doesn't mean warmup is on. Check that warmup emails are being sent and received.

• DNS not ready. If SPF, DKIM or DMARC haven't propagated when warmup starts, it can fail quietly. New records can take up to about two days.

• Cheap extensions. .info and similar domains tend to need a couple of weeks longer.

• Abrupt changes. Jumps in warmup or cold volume set you back. Change volume in steps.

Is day 14 always enough?

It's the minimum. Before you raise cold volume, check:

1. Warmup inbox rate is stable across several days and not falling. See warmup inbox rate dropped.

2. SPF, DKIM and DMARC pass on a test message.

3. A placement test with a neutral email lands in the inbox for both Google and Outlook recipients.

If any of these fail, keep warming and hold cold volume. If the Google score has stayed flat in the 60s or 70s for a week while Outlook is fine, waiting longer rarely helps: check the sending server first.

What warmup does not do

A high warmup score does not prove cold placement. Warmup partners have a history with your mailbox; your prospects don't. Use the warmup score as an early warning, and use placement tests and replies to judge cold email. See placement tests.

What mailbox vendors ask for

Maildoso's terms require at least 14 days of warmup in an outside tool before the first campaign (Maildoso review). Mailpool recommends three to four weeks (Mailpool review). Zapmail and Primeforge sell pre-warmed mailboxes that skip the wait for a higher monthly price (Zapmail review, Primeforge review). Outreach2day sells pre-warmed SMTP mailboxes too, available now. Skipping the wait means buying pre-warmed inboxes.

Plan ahead

Every new mailbox needs the full warmup, including the ones you add to replace a burned domain. If you plan to scale or rotate domains, create and warm the extra mailboxes two weeks before you need them.

Questions

How long does it take to warm up a new email domain?

A new domain warms up through its mailboxes: 14 days of warmup before the first cold email, then full volume of up to 25 cold emails per mailbox a day from day 14. Cheaper extensions such as .info can need a couple of extra weeks. After the first month domain age barely mattered in our data, so the 14 days of warmup are the part to plan for. Keep warmup running after you start sending.

What we recommend at Outreach2day

Warmup only for the first 14 days, full volume of up to 25 cold emails a mailbox a day from day 14, and warmup kept running while you send. New mailboxes are ready within 24 hours, and warmup is included for as long as you keep the mailbox.