How to Warm Up a New Email Domain for Cold Outreach (Step-by-Step)

A new email domain and mailbox have no sending reputation. When you start sending cold emails from a brand-new inbox, inbox providers like Google and Microsoft have no data to confirm that you're a legitimate sender. Without that history, your emails are far more likely to land in spam — sometimes for every single recipient.

Email warm-up is the process of building that reputation before you start sending campaigns. Done correctly, a properly warmed-up inbox delivers the majority of cold emails to the primary inbox from day one of your campaign. Skip it, and you risk burning a domain that took weeks to set up and hundreds of dollars to maintain.

What Email Warm-Up Actually Does

Email providers evaluate new mailboxes based on behavioral signals: how many emails they send, what percentage get opened, what percentage bounce, how many recipients mark them as spam, and whether recipients actively engage (reply, forward, move from spam to inbox). A brand-new mailbox has none of these signals — it's a blank slate, which makes it suspicious.

Warm-up tools simulate the behavioral patterns of a legitimate, actively-used email account. They send emails to a network of seed accounts, which automatically open them, reply to them, and — critically — move any that land in spam back to the inbox. This signals to Google and Microsoft that your mailbox is legitimate and that recipients value receiving email from you.

The result is a mailbox with a positive engagement history before your first campaign ever sends. That history dramatically improves inbox placement rates.

Step 1: Domain and DNS Setup First

Before warming up any mailbox, confirm your DNS records are correctly configured. SPF, DKIM, and DMARC records must be set and verified before warm-up traffic starts. Sending warm-up emails without proper authentication is like building a foundation on sand — the reputation you build won't stick because providers will still flag your authentication as incomplete.

Verify your DNS setup using MXToolbox or the built-in diagnostics in your warm-up tool. All three records (SPF, DKIM, DMARC) should show green before proceeding. For exact DNS configuration instructions, see the cold email deliverability checklist.

Step 2: Choose a Warm-Up Tool

The three most widely used warm-up tools in 2026 are Instantly, Lemwarm, and Mailreach. Each has a large network of seed accounts for warm-up traffic and provides deliverability monitoring dashboards.

• Instantly Warmup (included in Instantly plans): Best if you're already using Instantly for sending. Seamless integration, large warm-up network, and the dashboard shows deliverability metrics alongside your campaign stats.
• Lemwarm ($29/mailbox/month): Standalone warm-up tool with detailed deliverability reporting. The warm-up network is slightly smaller than Instantly's but the analytics are excellent. Good choice if you use a different sending tool.
• Mailreach ($25/mailbox/month): Focused specifically on warm-up with strong spam placement monitoring. Provides a deliverability score you can use to decide when a mailbox is ready for cold campaigns.

Avoid "free" warm-up tools with small networks. The quality and size of the warm-up network directly impacts how fast and reliably your reputation builds.

Step 3: The Warm-Up Schedule — Week by Week

Here is the recommended warm-up schedule for a new Google Workspace or Microsoft 365 mailbox. Follow this for each mailbox individually.

• Week 1 (Days 1-7): Start warm-up tool at 5 emails/day. Zero cold sending. Monitor spam placement rate — target below 5%.
• Week 2 (Days 8-14): Increase warm-up to 10 emails/day. Send 0-5 cold emails/day to very clean, verified lists only. If spam placement is above 10%, stay at 5/day for another week.
• Week 3 (Days 15-21): Warm-up at 15 emails/day. Cold sends at 10-15/day. Run a Glockapps deliverability test to check inbox vs spam placement across providers.
• Week 4 (Days 22-28): Warm-up at 15-20 emails/day. Cold sends at 20-25/day. Confirm bounce rate below 2% on cold sends. Check reply rate as a secondary engagement signal.
• Week 5 (Days 29-35): Warm-up at 20 emails/day (maintain indefinitely). Cold sends at 30-40/day. Full campaign launch.

Never stop the warm-up process just because you've started sending cold emails. Keeping warm-up running continuously maintains your mailbox reputation between campaign periods and during volume spikes.

What to Monitor During Warm-Up

These are the key metrics to watch throughout the warm-up period. Check them at least every 3 days.

• Spam placement rate (target: under 10%): Your warm-up tool shows what percentage of warm-up emails land in spam. If this number climbs above 15%, pause warm-up and check for DNS errors or other deliverability issues.
• Open rate on warm-up emails: Should be consistently high (80%+) since seed accounts are designed to open everything. If open rates drop, the warm-up network may have issues.
• Mailbox health score: Tools like Mailreach provide a composite health score. Target above 85/100 before starting full cold campaigns.
• Authentication verification: Re-run MXToolbox every 2 weeks to confirm DNS records haven't been accidentally altered by DNS cache changes or registrar edits.

Common Warm-Up Mistakes That Ruin New Domains

These are the most frequent errors that prevent warm-up from working — or actively damage mailbox reputation before campaigns even begin:

• Sending cold emails before DNS is verified. Without SPF/DKIM/DMARC, authentication failures immediately signal spam to providers.
• Sending cold emails before completing at least 2 weeks of warm-up. Even a clean list will produce spam placement if the mailbox has no reputation history.
• Ramping volume too fast. Going from 5 to 50 emails/day in a week triggers spam detection algorithms that look for sudden volume spikes from new mailboxes.
• Sending to unverified lists during warm-up. A hard bounce on a new domain immediately sets back reputation building. Only send to verified, clean lists during the warm-up period.
• Stopping warm-up after starting cold sends. The warm-up traffic provides ongoing positive engagement signals. Turning it off removes that protective layer.
• Using the same domain for marketing emails and cold outreach. Marketing automation tools like Mailchimp or HubSpot have very different sending patterns from individual outreach. Keep them on separate domains.

How Long Does Warm-Up Take?

The minimum warm-up period before running full cold campaigns is 3-4 weeks for most mailboxes. However, these factors require extending the warm-up period:

• Domain age under 30 days: Add an extra 1-2 weeks. Very new domains are scrutinized more heavily.
• High spam placement rate (>10%) during warm-up: Do not escalate volume until spam placement drops and stabilizes below 5%.
• Previous deliverability issues on the domain: If a domain was previously used for cold email and had problems, a standard 4-week warm-up may not be sufficient. Consider purchasing a new domain instead.
• Microsoft 365 mailboxes: Generally take 1-2 weeks longer to warm up than Google Workspace for equivalent deliverability.

For a complete guide to building your full sending infrastructure, see how to set up cold email infrastructure that avoids spam and how many cold emails to send per day.

Warm-Up for Domains That Have Been "Burned"

If you have an existing domain that was previously used for cold email and is now showing poor deliverability — high spam placement, low open rates, or accounts flagged by Google — the warm-up process is significantly longer and less reliable. In most cases, it's faster and more effective to purchase a new domain and build reputation from scratch than to try to recover a damaged one. The cost of a new domain is $10-15. The cost of running campaigns with 20% inbox placement for months while trying to recover a burned domain is much higher.

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