What warmup actually does, the tools that do it, how long it takes, and the signs your warmup is working vs. quietly failing.
Rees Bayba
Founder, Astra GTM
TL;DR
A fresh email mailbox has no sending history. Email providers -- Gmail, Outlook, Yahoo -- treat it with suspicion. Before it has established a track record of normal, legitimate email activity, any high-volume sending triggers spam filters. Warmup is the process of building that track record before you start cold outreach. Skip it and your first campaign goes straight to spam, burns your domain, and digs a reputation hole that takes months to climb out of.
Warmup tools operate by connecting your new mailbox to a network of real inboxes. Your mailbox sends emails to other inboxes in the network. Those inboxes automatically open the emails, reply to them, and -- critically -- rescue any that land in spam by moving them to the inbox. This simulates legitimate human email behavior at a scale that builds sender reputation quickly.
Never warm up your primary business domain. If something goes wrong -- a warmup misconfiguration, a burst of spam complaints, an aggressive sending tool -- you want the damage isolated to a sending domain, not the domain tied to your transactional email, your website, and your brand.
This step comes before creating a mailbox, before connecting to a warmup tool, before anything. SPF, DKIM, and DMARC must be in place before warmup starts. Warming a mailbox with misconfigured DNS does nothing useful -- the warmup emails have no authentication and cannot build legitimate reputation. Come back to this step last and you have wasted weeks.
SPF tells receiving servers which mail servers are authorized to send email from your domain. Add a TXT record: Host @ | Value: v=spf1 include:_spf.google.com ~all (for Google Workspace). If you are using Microsoft 365, use include:spf.protection.outlook.com instead.
DKIM adds a cryptographic signature to every email. Google Workspace generates the DKIM key in your admin console (Apps → Google Workspace → Gmail → Authenticate email). Copy the key value and add it as a TXT record in your domain's DNS. Microsoft 365 does the same through the Microsoft 365 admin center.
DMARC tells receiving servers what to do when SPF or DKIM fail. Start with p=none (monitoring only): Host _dmarc | Value: v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.com. After 2-4 weeks of clean reports, upgrade to p=quarantine.
Use Google Workspace ($7.20/mailbox/month) as your primary sending platform. Microsoft 365 inbox placement for cold outbound dropped to 26.77% in Q1 2025 -- use Google unless you have a specific reason to target Outlook-heavy markets. Create 2-3 mailboxes per domain.
Three tools dominate the warmup market. All work on the same principle -- a network of inboxes that send, open, reply to, and rescue each other's emails. They differ in pricing, network size, and platform integration.
| Tool | Price | Network size | Best for |
|---|---|---|---|
| Instantly Warmup | Included with Instantly plans ($37-97/mo) | 300,000+ mailboxes | Teams already using Instantly for campaigns -- warmup and sending in one platform |
| Mailwarm | $49/mo per mailbox | 15,000+ mailboxes | Standalone warmup, works with any sending platform |
| Warmbox | $15/mo per mailbox | 35,000+ mailboxes | Budget-conscious setups, good for high mailbox counts |
If you are already using Instantly or SmartLead for campaign sending, use their built-in warmup. It saves cost and keeps all your mailbox management in one place. If you are on a different sending platform, Warmbox gives the best cost-to-quality ratio for standalone warmup.
The warmup ramp follows the same logic every time: start low, increase gradually, do not rush. Email providers become suspicious when a new mailbox sends at high volume immediately. A slow ramp looks like a normal person who is getting more active with email -- which is exactly what you want.
| Day range | Warmup emails/day | Cold emails allowed |
|---|---|---|
| Days 1-7 | 3-5 | 0 -- do not touch this mailbox for cold sends yet |
| Days 8-14 | 10-15 | 0 -- still warming, no cold sends |
| Days 15-21 | 20-30 | 0-5 (conservative start only if warmup metrics look strong) |
| Days 22-28 | 30-40 | 10-20 (alongside continued warmup) |
| Day 29+ | 30-50 (steady state) | 30-50 (full campaign volume) |
Target 30-50 warmup emails per day at steady state. Keep warmup running continuously -- it is not a phase you complete and turn off. Think of it as ongoing reputation maintenance, not a one-time setup. When you pause campaigns for weekends or holidays, warmup should keep running.
14 days is the absolute minimum. 21-28 days is better. If your domain is brand new (registered in the last week), add 3-5 days to the timeline. Rushing warmup is the most common reason campaigns fail from day one.
Google Postmaster Tools is a free dashboard that shows your domain's sending reputation from Google's perspective -- the most important perspective for cold email. Set it up on day 1 and check it weekly.
Warmup running in the background is easy to ignore. These are the signals that tell you whether it is building reputation or quietly failing.
| Signal | Warmup working | Warmup failing |
|---|---|---|
| Inbox placement in warmup dashboard | 80%+ by week 3 | Below 60% after 2 weeks |
| Google Postmaster domain reputation | Medium or High by week 3 | Stuck at Low after 3 weeks |
| Spam rate (Postmaster) | Below 0.08% | Above 0.1% and rising |
| Test email to personal Gmail | Primary inbox | Spam or Promotions |
| Warmup reply rate | 40-60% of warmup emails get replies | Below 20% reply rate |
Do I need to warm up every new mailbox, even if the domain is already warmed?
Yes. Domain reputation and mailbox reputation are separate signals. A new mailbox added to an established domain still needs its own warmup. The domain's history helps, but the specific mailbox address has no sending history of its own. Run the full warmup protocol for every new mailbox.
How long does warmup take for a completely new domain?
For a brand new domain (registered in the last week), add 3-5 days to the standard timeline. Domains with zero history start further back in terms of trust than a domain registered 6 months ago with no sending history. Standard target: 21-28 days for a new domain before cold sends.
Can I run warmup and cold email campaigns at the same time?
Yes, but only after the initial warmup period is complete. Once your mailbox has reached 30-50 warmup emails per day and shows good reputation metrics (domain reputation Medium/High, spam rate below 0.1%), you can begin adding campaign sends while keeping warmup running. Start conservatively: 10-15 cold sends per day for the first week of simultaneous operation.
What happens if I stop warmup?
Sender reputation is not permanent. If you stop warmup for 2-3 weeks, your reputation begins to decay. After 4-6 weeks without any sending activity, a mailbox can drop back to an unknown status with email providers. If you stop warmup for more than 2 weeks, treat the restart like a fresh setup -- 14-21 day ramp before resuming campaign sends.
Is warmup necessary if I'm using a well-established personal domain?
If you have been actively using a domain for years with consistent legitimate email activity, warmup is less critical. But even established domains need warmup when you add new mailboxes or significantly increase sending volume. Moving from 5 emails/day to 50 emails/day on any mailbox requires a gradual ramp, regardless of the domain's history.
How do I know when warmup is done and I can send at full volume?
Three signals together: (1) Google Postmaster shows your domain at Medium or High reputation, (2) your warmup dashboard shows inbox placement above 80%, (3) a test email to a personal Gmail account lands in the primary inbox. When all three are true after at least 14 days of warmup, you can start cold sends. Keep warmup running indefinitely -- there is no finish line.
We implement these systems end-to-end. First sends within 14 days.