Quick answer
Buy separate domains for cold email and never send it from your main one. Put 2 or 3 mailboxes on each domain, set up SPF, DKIM and DMARC, and connect every mailbox to a warmup tool. Let it run for 3 to 4 weeks before any real prospect gets an email. Then start at 20 to 30 cold emails per mailbox per day and keep warmup running in the background. Watch bounces, spam complaints and placement tests, and when you need more volume, add domains instead of pushing each mailbox harder.
What warmup does, and what it can't do
I set up outbound for B2B companies, and my rule for every new domain is the same: warm up for 3 to 4 weeks, then send 20 to 30 emails per mailbox per day. Here is why it takes that long.
A new domain has no sending history, and Gmail, Outlook and Yahoo are careful with senders they don't know. Google's sender guidelines say it plainly: send at a consistent rate, and avoid sudden volume spikes if you don't have a history of sending large volumes. Warmup builds that history. A warmup tool sends a few emails a day between your mailbox and other mailboxes in its network, and those mailboxes open them, reply and pull them out of spam. Over a few weeks the volume climbs and your domain starts to look like a normal sender.
What warmup can't do: fix missing DNS records, a bad list or copy that reads like a promotion. And no tool guarantees the inbox. Deliverability is a grey zone, and anyone who says they get 100% is lying.
Step 1: Buy separate sending domains
Cold email carries risk: a bad list or a spike in spam complaints hurts the domain it came from. Your main domain carries your invoices, support mail and every reply from customers, so keep cold email off it.
Even Microsoft tells companies to move bulk mail they don't control off the main domain and onto a subdomain (Microsoft). For cold email, most teams go one step further and buy separate domains. One reason: Gmail counts mail from subdomains toward the main domain when it decides who is a bulk sender (Google).
- Pick names close to your brand. If your company is example.com, something like getexample.com, tryexample.com or examplehq.com. Prospects see the domain, so it should look like you.
- Point each one at your main website, so anyone who types it in lands somewhere real.
- Buy one or two spares and warm them with the rest. When a domain has a bad week, the replacement is already warm.
Step 2: Decide how many mailboxes you need
Work backwards from the number of cold emails you want to send a day. At 20 to 30 per mailbox and 2 or 3 mailboxes per domain, the maths looks like this:
| Cold emails a day | Mailboxes (at 25 each) | Domains (at 3 mailboxes each) | Plus spares |
|---|---|---|---|
| 100 | 4 | 2 | 1 domain |
| 300 | 12 | 4 | 1 or 2 domains |
| 600 | 24 | 8 | 2 domains |
Clay's own guidance lands in the same place: about 20 emails per day per inbox and 3 inboxes per domain, 5 at the most (Clay). Give every mailbox a real first name, a photo and a signature. An address like sales@ or info@ looks like automation to filters and to people.
You can host the mailboxes on Google Workspace or Microsoft 365, or with a provider built for cold email that sets up the DNS records for you. I go through those options in cold email infrastructure services.
If you would rather not do the maths by hand, the free domains and inboxes calculator on Shipyard GTM, my agency's site, works it out from your monthly volume.
Step 3: Set up SPF, DKIM and DMARC before the first warmup email
Every sending domain needs all three records before a single email goes out, warmup emails included. Without them the warmup emails themselves can land in spam, and that's the opposite of what you're building.
- SPF: one record listing your mailbox provider, for example
v=spf1 include:_spf.google.com ~allon Google Workspace. - DKIM: a key generated by your mailbox provider for your own domain, 2048 bits if your DNS host supports it.
- DMARC: at least
v=DMARC1; p=none; rua=mailto:dmarc-reports@example.com.
Run each domain through the free cold email deliverability checker before you connect it to warmup. The full setup, with the mistakes that break it, is in SPF, DKIM and DMARC for cold email.
Step 4: Follow a 3 to 4 week ramp
Connect every mailbox to a warmup tool and leave its default ramp for new accounts on. Most tools start very low and add a little each day. Instantly's suggested setting for a new account, for example, starts at 1 warmup email a day and adds 1 a day up to 10 (Instantly). Here is the week-by-week plan:
| Week | Warmup | Cold emails per mailbox per day | Check before moving on |
|---|---|---|---|
| 0 | Buy domains, create mailboxes, add DNS records, connect warmup | 0 | SPF, DKIM and DMARC all pass |
| 1 | Tool's starting ramp | 0 | No bounces or connection errors in the tool |
| 2 | Climbing | 0 | Warmup emails landing in the inbox, not spam |
| 3 | Full warmup volume | 0 | A placement test from each domain |
| 4 | Full warmup volume | 0, or about 20 if every week 3 check passed | Verified list and final copy ready |
| 5 on | Keeps running in the background | 20 to 30 | Bounces, spam complaints and replies on real sends |
Other people's numbers, so you can compare. These are their claims, not mine:
- Instantly's slow ramp guide (October 2025): at least 14 days of warmup before meaningful volume, 21 to 28 days is safer, and 30 a day per inbox as the cap for most B2B cold outreach (Instantly).
- Instantly's domain warmup guide (updated June 2026): after three to four weeks you can start cold outreach, and keep around 20% to 30% of volume for ongoing warmup (Instantly).
- Clay: mailboxes bought through Clay take roughly three weeks to become fully warmed (Clay).
The numbers differ a little, but nobody serious says one week.
Stick to the ramp. A fresh domain that goes from warmup to 200 cold emails in one day is exactly the sudden volume spike Google's guidelines warn about. When you need more volume, warm more domains and mailboxes instead of pushing the ones you have.
Step 5: Pick a warmup tool
If you already send with a cold email tool, start with the warmup it includes. A separate tool makes sense when you want warmup that stays put if you switch senders or placement tests on top. Prices change, so check each vendor's current pricing.
| Tool | What it says it does | Pick it if |
|---|---|---|
| Warmforge | Warmup pool made up "exclusively of aged accounts", mostly Google Workspace and Microsoft 365. One free warming slot on your first workspace and one free placement test a month. Unlimited warmup is included with Salesforge plans | You send with Salesforge, or want warmup and placement tests in one place |
| Instantly | "Unlimited Email Warmup" on its Outreach plans | You send with Instantly |
| Smartlead | Advertises unlimited warmups and a warmup pool | You send with Smartlead |
| lemwarm (lemlist) | Included in all lemlist plans | You send with lemlist |
| MailReach | Warmup for any provider that supports SMTP, plus a spam checker for placement tests | You want warmup separate from your sending tool |
Sources: Warmforge, Salesforge pricing, Instantly pricing, Smartlead pricing, lemlist pricing and MailReach, all checked on 14 October 2026.
What I run. For clients I use Mailforge or Infraforge for mailboxes and Warmforge for warmup, all part of Salesforge's Forge stack, and I've written guest articles for Salesforge's blog. No tool in this article pays me and there are no affiliate links. That's a preference from daily use, not a verdict. If you already send with Instantly, Smartlead or lemlist, their built-in warmup is the simpler choice, and MailReach is a fair pick if you want warmup that doesn't depend on your sender.
Some providers also sell mailboxes that come "pre-warmed". They can save time, but your own sending pattern still starts from zero, so start real sends at the low end and watch the first two weeks closely.
Step 6: Decide when to start sending
Time alone isn't the signal. Start real sends when all of these are true:
- The domain has had at least 3 weeks of warmup, 4 if anything looked off.
- SPF, DKIM and DMARC pass on every sending domain.
- A placement test shows most of your emails in the inbox at Gmail and Outlook. Instantly's slow ramp guide, for comparison, waits for seed placement of 80% or more before moving on.
- The list is verified, so bounces stay low from the first send.
- The copy is short, plain text and light on links.
Then start at about 20 cold emails per mailbox per day with warmup still running. Move toward 30 only while bounces and complaints stay low.
Step 7: Monitor every domain after launch
| Signal | Where to see it | What to do |
|---|---|---|
| Bounce rate | Your sending tool | Instantly's guide pauses a mailbox when bounces go over 1%. Check the list before sending more |
| Spam complaints | Google Postmaster Tools, your sending tool | Google wants you under 0.1% and never at 0.3% or higher |
| Placement | Placement tests in your warmup tool or a seed list | If a domain starts landing in spam, stop cold sends on it and keep warmup running |
| Reply rate | Your sending tool | A drop with no change in list or copy usually means deliverability |
Set up Google Postmaster Tools on every sending domain. It shows the spam rate Gmail users report and whether you meet Gmail's sender rules, though on low-volume days it may not show all data. Google has said its domain and IP reputation dashboards will be retired, so don't build your monitoring around them.
When a domain goes bad, don't push through it. Stop cold sends on it and keep warmup running. Check its DNS and the list it was sending to, then move its volume to a spare. If placement doesn't come back, retire it from cold email. The inbox placement audit checklist walks through the full check.
Key takeaways
- Send cold email from separate domains close to your brand, never from your main domain.
- Plan 2 or 3 mailboxes per domain and work out the count from your daily volume, plus spare domains.
- Set up SPF, DKIM and DMARC before the first warmup email.
- Warm up for 3 to 4 weeks, then send 20 to 30 cold emails per mailbox per day with warmup still running.
- Use the warmup built into your sending tool unless you need it to work independently.
- Watch bounces, spam complaints and placement on every domain, and add domains to grow volume.
FAQ
How long does it take to warm up an email domain?
Plan for 3 to 4 weeks before a new domain sends to real prospects. Instantly's guide says at least 14 days, with 21 to 28 days safer, and Clay says roughly three weeks for mailboxes bought through it.
How many emails should I send per day after warmup?
Start at about 20 cold emails per mailbox per day and go up to 30 while bounces and complaints stay low. To send more, add mailboxes and domains instead of raising the number per mailbox.
Can I send cold emails while the domain is warming up?
Not in the first 3 weeks. Real prospects bring bounces and spam clicks that a new domain can't absorb yet. Once you do start, keep warmup running in the background.
Do I need a warmup tool, or can I warm up a domain manually?
You can warm up by emailing people who will open and reply, but it's slow and hard to do for more than a few mailboxes. Most cold email tools include warmup, so in practice a tool is the easier route.
How many mailboxes should I have per domain?
2 or 3 is the common setup. Clay recommends 3 inboxes per domain and no more than 5.