Email Warmup: How Long to Warm Up a New Sending Domain (and How to Know It Worked)

TLDR

After a bounce spike on one of my cold email campaigns, I fixed the cause and ran inbox placement tests again. Microsoft inboxes took the email: 4 of 4 in the inbox. Google took 2 of 8. Both sending domains passed SPF, DKIM and DMARC and sat on no blocklist. Warmup had been running the whole time.

The direct answer: plan 4 to 6 weeks between buying a new sending domain and its first real campaign send. Warm it up with steady daily volume, then treat a passed inbox placement test for each email provider (Google and Microsoft separately) as the finish line, not the calendar. Warmup builds a starting reputation. It cannot repair one after a bounce spike.

Email warmup is the period when a new sending domain and its inboxes build a sending history before they send cold email at volume. Every cold email guide mentions it. Few say how you know it worked, and almost none say what happens when warmup finished but the inbox still says no. That last case is the one I lived through, so this page covers both.

How long does email warmup take?

For a brand-new domain, plan 4 to 6 weeks from purchase to first campaign send. Published advice ranges from 2 weeks to 3 months, and the real answer depends on when your placement tests pass for each provider, not on a fixed date.

Here is what the published sources say, so you can see the spread:

Two things stand out. First, every number comes from a company that sells warmup or sending tools; only Google’s guidance comes from an inbox provider, and Google gives a principle, not a timetable. Second, Smartlead’s own data says what senders do (8 days on average), and the page is clear that it does not compare results by warmup length, so it can’t show that 8 days is enough.

My 4 to 6 weeks is longer than most of these because it counts from purchase, includes setup and verification, and assumes the domain must pass a placement test at both Google and Microsoft before it sends anything real.

Why is 4 to 6 weeks my planning number?

Because the steps before the first real send take that long when you do them in order and verify each one: buying aged-looking domains, setting up authentication, warming, and testing placement per provider.

The timeline I plan for each new sending domain:

1. Week 0: buy and set up. Buy the domain, point it to the real website (sending domains carry no content of their own), create the inboxes, publish SPF, DKIM and DMARC, and verify each with a lookup rather than trusting the DNS panel. I once found a domain with no DKIM published while its panel said email was configured.
2. Week 0 to 1: custom tracking domain and blocklist check. One tracking domain per sending domain, so a bad campaign on one can’t drag the others down. Check the domain against Spamhaus and a multi-list blacklist check.
3. Weeks 1 to 5: warmup at steady volume. Warmup runs every day. Consistency matters more than size here; Google says consistent volume “is particularly important for new domains as they establish their sending reputation” (Google).
4. Weeks 4 to 6: placement tests, per provider. A seed test to Google inboxes and to Microsoft inboxes, read separately. The domain sends real campaigns only to the provider it passed.
5. First sends: low volume, then steps. Google’s guidance for raising volume is a common daily increase of 25% to 100%, holding steady for 24 hours after any deferrals (Google).

The practical consequence: if a program needs to start sending in a month, the domains should have been bought last month. I buy them ahead for that reason.

Why doesn’t a good warmup score mean you’ll reach the inbox?

Because warmup emails travel between accounts in a warmup network that are set up to open, reply and pull messages out of spam. They land in the inbox almost every time, so the score measures the network, not your real recipients.

Smartlead reports that warmup emails reach the inbox about 99% of the time (Smartlead). That is useful for keeping the inbox active. It tells you very little about whether a cold email to a new prospect at a Google Workspace company will land. For that you need a placement test: send the actual campaign email to a set of seed inboxes at each provider and count where it lands.

What happened when warmup wasn’t enough?

A campaign of mine paused itself after a bounce spike, and after I fixed the cause, Google still kept most of the email out of the inbox while Microsoft accepted it. Warmup had been running the whole time.

The situation: a cold email campaign to financial advisors auto-paused after 138 hard bounces on 1,440 sends. The bounce reasons were 74% reputation blocks, 17% policy blocks and only 8% invalid addresses, so the list wasn’t the cause. The block came from the sending provider’s own outbound filter rejecting the shared open-tracking domain the email platform puts in every message. I put a custom tracking domain on each sending domain, published a missing DKIM record and cleared a reply-to on an unrelated domain.

Then I ran placement tests:

By domain at Google, one sending domain landed 0 of 9 and the other 2 of 5, and only with tracking off. Authentication passed everywhere, and neither domain was on a blocklist. These are small seed tests, so I read them as direction, not precise rates.

My reading: the bounce spike most likely sank both young domains’ standing with Google, while Microsoft kept accepting the mail. Fixing the cause did not restore it, and continued warmup did not either, at least not within the days I watched. Warmup builds a starting reputation on a clean domain. It is not a repair tool for a domain that just had a bad week.

What I changed because of it:

• Every campaign is now planned per recipient provider. Leads are tagged Google, Microsoft or other from their MX record before the first send.
• Google Workspace inboxes send to Google-hosted recipients and Microsoft 365 inboxes send to Microsoft-hosted recipients, on separate domains, with the same copy.
• Each domain must pass a placement test for the provider it will send to. A domain can be cleared for Microsoft and held back from Google.
• A text-only, tracking-off rerun is the first isolation test when placement drops, because it separates content and tracking from domain reputation.

How do you know warmup is done?

Warmup is done for a provider when a seed placement test of your real campaign email lands mostly in the inbox at that provider, with authentication passing and no blocklist hits. Until then, keep warming and keep volume low.

The checklist I use before a domain sends its first real campaign to a provider:

• SPF, DKIM and DMARC verified with a lookup (not just the DNS panel)
• Custom tracking domain verified, or tracking off
• Reply-to blank or on the same domain as the sender
• Domain clean on Spamhaus and a multi-list blacklist check
• At least 4 weeks of steady daily warmup
• Placement test passed at that provider with the real email
• For Google-side domains, Google Postmaster Tools set up so spam rate can be watched once volume is high enough to report

What should you avoid during warmup?

• Jumping volume. Google warns that “increasing your sending volume too quickly can result in delivery issues” and advises steady sending rather than bursts and pauses (Google).
• Sending cold email from your main domain. Use separate sending domains so your day-to-day email is protected.
• Putting content on sending domains. Domain reputation comes from sending behavior, not from a website. Sending domains only redirect to the real site.
• Judging by the warmup dashboard. Judge by placement tests and, later, by bounces, complaints and replies.
• Treating Google and Microsoft as one inbox. In my tests they disagreed about the same email from the same domains.

FAQ

How long should I warm up a new email domain before cold outreach? Plan for 4 to 6 weeks from purchase to the first real campaign send on a brand-new domain. Vendor guides range from 2 weeks to 3 months. The end of warmup is a passed inbox placement test for each email provider you will send to, not a date.

Can I send cold email while the domain is warming up? Only at low volume and only after a placement test passes for that provider. Keep the warmup running alongside, raise volume in steps, and stop raising it the moment bounces or complaints move.

Does a high warmup score mean my emails will reach the inbox? No. Warmup messages go between warmup accounts that are built to open and rescue them, so they land in the inbox almost every time. Real placement has to be tested with a separate seed test to Google and Microsoft inboxes.

Do I need separate domains for Gmail and Outlook recipients? I plan them separately. Google and Microsoft judged the same email from the same domains very differently in my own tests, so I send to Google-hosted recipients from Google Workspace inboxes and to Microsoft-hosted recipients from Microsoft 365 inboxes, on separate domains.

Can warmup fix a domain that got flagged? Not quickly. After a bounce spike, fixing the cause did not restore one of my domains’ standing with Google. Plan for weeks of repair, or retire the domain from Google-side sending and use a fresh, already-warmed one.

How many emails per day should a warmed-up inbox send? Keep cold sends modest per inbox and grow total volume by adding inboxes and domains, not by pushing each inbox harder. Google’s own guidance for raising volume is a 25% to 100% daily increase from a steady base, slowing down at the first deferrals.

Check your domain before you warm it

Warmup on a domain with a broken record wastes the weeks. Run the free SEO and domain scan to check SPF, DKIM, DMARC and the transport records first, and read the cold email deliverability setup guide for the DNS steps. If you want domains bought, warmed and tested per provider for you, that is our cold email service.

Sources

• Google, Top 10 Gmail sender issues
• Google, Email sender guidelines
• Google, Set up Postmaster Tools
• Smartlead, How long to warm up email before cold outreach
• Warmy, How long to warm up new domains before starting to send campaigns
• Our own campaign data: bounce analysis and per-provider placement tests on a live cold email campaign (anonymized)