Email Warming: What It Is, Why You Need It, and How to Do It Right
June 13, 2026
Email warming is the process of gradually increasing sending volume from a new domain or mailbox so that mail providers (Gmail, Outlook, Yahoo) build a positive sender reputation. Without warming, emails from a new domain land in spam even with perfect technical configuration.
Email warming is the process of gradually increasing sending volume from a new domain or mailbox so that mail providers (Gmail, Outlook, Yahoo) build a positive sender reputation. Without warming, emails from a new domain land in spam even with perfect technical configuration.
Why Warming Is Necessary
A new domain has no sending history. From Gmail's or Outlook's perspective, that's suspicious — most spammers register new domains and immediately start mass sending.
Warming solves three problems:
- Builds a sending history with high engagement (opens, replies)
- Signals to providers that the domain is being used for real communication
- Allows you to gradually reach working volume without triggering spam filters
According to Warmup Inbox, domains without warming achieve an average deliverability of 45% in the first month. After proper warming — 92–98%.
How Warming Works Technically
During warming, a mailbox automatically sends and receives emails from other mailboxes in the warming network. These emails are opened, replied to, and rescued from spam — creating the pattern of an active, trusted sender.
What happens under the hood:
- The mailbox sends emails to other participants in the warming network
- Recipients open the emails and reply to them
- If an email lands in spam, it gets marked as "Not spam"
- Volume gradually increases: 20 → 50 → 100 → 200 emails/day
Warming Schedules: Two Approaches
Manual Warming (slower, but free)
| Week | Emails per day | Actions |
|---|---|---|
| 1 | 5–10 | Emails to colleagues, partners, acquaintances |
| 2 | 15–20 | Expand the list, ask for replies |
| 3 | 25–40 | First test emails to new contacts |
| 4 | 50–70 | Small campaigns with a quality list |
| 5–6 | 100–150 | Approaching working volume |
| 7+ | 200–300 | Full campaigns |
Automatic Warming (faster, recommended)
With warming tools (Instantly, Mailwarm, Warmup Inbox, built-in warming in Reply.io):
| Week | Emails per day | Actions |
|---|---|---|
| 1–2 | 20–40 | Automated email exchange within the network |
| 3–4 | 60–100 | Volume increases automatically |
| 5–6 | 150–200 | Ready to launch working campaigns |
The advantage of auto-warming: a network of hundreds of mailboxes simulates real user behavior that algorithms cannot distinguish from organic activity.
Readiness Checklist Before Launching Campaigns
Before starting a campaign, confirm:
What Kills a Warm-Up
Mistakes that undo all your progress:
- Sudden volume spike — going from 50 to 500 emails in a single day → immediate red flag
- Launching a campaign to a low-quality list — high bounce rate in the first days destroys reputation
- Pausing warming for 2+ weeks — reputation degrades and you have to start over
- Sending spam-trigger content — filters train on content, not just volume
- Warming without DKIM enabled — unsigned emails don't build DKIM domain reputation
Warming Tools: Comparison
| Tool | Type | Network size | Cost |
|---|---|---|---|
| Instantly | Built-in | 500,000+ mailboxes | Included in Instantly plans |
| Warmup Inbox | Standalone service | 30,000+ mailboxes | $15–49/month per mailbox |
| Mailwarm | Standalone service | 5,000+ mailboxes | $49–249/month |
| Reply.io | Built-in | Proprietary network | Included in Reply.io |
| Manual | No service | — | Free, but slow |
Warming After a Burned Domain
If a domain has received a high spam rate or been blacklisted, warming will not restore it. You need a new domain.
The rule: a burned domain cannot be revived through warming. Warming only works at the start — it's not a cure.
When switching to a new domain:
- Register an alias domain
- Configure SPF, DKIM, DMARC
- Start warming from scratch
- Forward replies from the old mailbox via email forwarding
HolyMailer creates new domains with pre-configured DNS records and ready-to-use mailboxes in 10 minutes — manual setup of a new domain takes an average of 32 hours.
FAQ
Is warming needed for Google Workspace or Microsoft 365?
Yes. Warming is required regardless of the provider. Google Workspace and Microsoft 365 provide technically sound infrastructure, but not domain reputation — that has to be earned gradually.
Can I run real campaigns during warming?
Yes, but only at low volume and with a quality list. Early campaigns during warming should not exceed 30% of your current warming volume.
How many mailboxes can be warmed simultaneously on one domain?
3–4 mailboxes per domain is the standard. More than that raises the risk to the domain's reputation if one mailbox runs into problems.
Does warming help for an existing domain?
Yes. If a domain hasn't been used for sending in a long time, it's best to start with low volumes and scale up gradually. A sudden restart after a long pause is just as much of a red flag as launching with a brand new domain.
How do I know if warming was successful?
The primary indicator is an inbox placement rate above 90% in tests via GlockApps or Mail-tester.com. Additionally, the domain should not appear on any blacklists, and the Spam Rate in Google Postmaster Tools should be below 0.1%.
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