Email warm up for cold outreach: authentication first, real conversations, a gradual volume ramp, and the checks that tell you the domain is ready.

What is email warm up?

Email warm up is the practice of sending a small and slowly growing volume of mail from a new domain and mailbox, and getting real replies to it, so mailbox providers have a record of normal behavior before your first cold email campaign goes out.

A domain you registered last week has no sending history anywhere. Gmail, Outlook.com and Yahoo have nothing to compare your traffic against, so the first few hundred emails are judged almost entirely on authentication, content and the reaction of the people who receive them.

Warmup is not a trick that makes filters friendly. It is the boring part of setup: get the DNS records right, send to people who will answer, keep volume low enough that a bad day cannot define your record, and watch the signals providers actually publish.

The order matters. Authentication comes first, because an email that fails it can be rejected outright. Real conversations come second, because engagement is what a new domain has none of. Volume comes last, and only when the first two hold.

You will see the practice written three ways: email warm up, email warm-up and email warmup. They describe one thing. Most tools and most cold outreach articles spell it as one word, and this guide uses warmup from here on.

Why a new domain and mailbox start at zero

Reputation in email is attached to things providers can verify: the sending IP address, the authenticated domain, and the history of how recipients treated mail from both. A new domain has none of that history, and a new mailbox on a shared provider inherits only the IP.

• No domain history: nothing ties your domain to past mail that people opened, answered or ignored.
• Shared IP, borrowed reputation: on Google Workspace or Microsoft 365 you send from shared infrastructure, so the IP reputation is not yours to build or ruin alone.
• Authentication does the talking: with no history, SPF, DKIM and DMARC are most of what a receiving server can check.
• Recipient reaction is the fastest signal: replies, moves out of spam and complaints all register quickly at low volume.
• Mistakes are expensive early: a burst of complaints against a tiny volume is a large percentage.

Google's own sender guidelines put it plainly for anyone growing traffic: start with a low sending volume to engaged users and increase it slowly over time, avoiding bursts and sudden spikes.

Domain reputation, IP reputation and what warmup can move

Two reputations decide how your emails are treated. Domain reputation follows the domain that authenticated the email. IP reputation follows the server that sent it. Warmup moves the first far more than the second.

If you run cold outreach through Google Workspace or Microsoft 365, the sending IP belongs to the provider and is shared with thousands of other senders. You cannot warm that IP. What you can build is your own domain reputation, and that is where warmup does its work.

Google's sender guidelines are direct about the shared case: the activity of any sender on a shared IP affects the reputation of everyone using it, and a negative reputation there can lower your delivery rate. Check the IP against blocklists as well as the domain.

Both reputations are rolling assessments of recent behavior. That is why a clean warmup is not a permanent credential. A domain with a spotless month can lose its inbox placement in one week of complaints, and a quiet domain slowly becomes an unknown one again.

Use a separate domain for cold outreach

Send cold outreach from a domain you can afford to lose, not the one your invoices and support mail travel on. A common pattern is a lookalike domain that redirects to the main site, with mailboxes created only for outbound.

One detail worth knowing: Gmail counts the 5,000 message daily threshold for bulk senders per primary domain, and messages from subdomains count toward that primary domain, according to the Gmail sender guidelines FAQ.

Point the sending domain at your real website and give it a real page. A domain that resolves to nothing, with a brand new registration and no content, is a weak signal to anyone checking who you are.

The DNS records to set before the first send

Warmup cannot fix a missing record. Everything below belongs in DNS before the mailbox sends its first email, because providers check these on every delivery attempt.

If you send through Google Workspace or Microsoft 365, the provider owns the sending IP and its PTR record, so your job is SPF, DKIM, DMARC and the MX records for the domain.

What Google, Microsoft and Yahoo actually require

These are published requirements, not folklore. Every line below comes from the sender guidelines the three providers maintain themselves.

Two things stand out for anyone warming up a domain. The authentication rules are not optional and not gradual, and the complaint thresholds are the only hard numbers any of the three publish.

Spam complaint rate, the one number providers publish

Gmail's sender guidelines tell all senders to keep the spam rate reported in Postmaster Tools below 0.3%. The sender guidelines FAQ goes further: keep the user-reported spam rate below 0.1% and prevent it from ever reaching 0.3% or higher.

That FAQ also explains the shape of the penalty. The impact is graduated, rates of 0.3% or higher have a greater negative effect, and rates above 0.1% already hurt inbox delivery. Spam rate is calculated daily.

Yahoo asks for the same 0.3% ceiling and adds a detail that changes the math: Yahoo calculates spam rate against mail delivered to the inbox, so your own numbers will not match unless you account for that.

During warmup the arithmetic is unforgiving. At forty emails a day, a single complaint is well above 0.3% for that day. That is an argument for warming up with people who expect your mail, not with strangers.

Setting up the mailbox so it looks used

A warm domain with an empty mailbox is half a setup. The mailbox itself carries signals, and most of them take ten minutes to get right.

• A real display name: a person's name, not a department or a slogan. Gmail's guidelines warn against display names that hide the sender or imitate a reply.
• A From: address that receives replies: Microsoft's recommendations ask that the From: or Reply-To address be valid, reflect the sending domain and accept replies.
• A plain signature: name, role, company and one link. No image-heavy block, no tracking pixel farm.
• Read the mailbox: connect it to a client over an IMAP server or the provider's own app, so replies get answered the same day.
• Working bounce handling: bounces come back to the domain, and Microsoft lists bounce management as a hygiene practice.
• An inbox that is not empty: subscribe to a few newsletters the recipient of your mail would also read.

None of these are magic. They just remove the reasons a filter has to treat the mailbox as disposable.

A sending volume ramp you can follow

No provider publishes a warmup schedule. Google says to start low, increase slowly, send at a consistent rate and avoid bursts, and that is as specific as the official guidance gets.

The warmup plan below was written for this page as a starting point for one mailbox on a new domain. It is not a benchmark, it is not published by any provider, and your own numbers should move with what you see in monitoring.

Two rules matter more than the exact numbers. Increase gradually rather than doubling, because Gmail warns that suddenly doubling previous volume can bring rate limiting or a reputation drop. And if messages start bouncing or getting deferred, reduce volume until the error rate falls, then climb again slowly.

Real conversations are the part that counts

Engagement signals that mean something come from people who wanted your message. During warmup you have a rare advantage: you can choose who receives the first few hundred messages, so choose people who will answer.

• Colleagues and partners: ask for a two-line reply, not a thumbs up. Threads with replies look different from one-way traffic.
• Existing customers and past contacts: a short, useful note from the new address, sent because there is a reason to write.
• Your own accounts: one address each at Gmail, Outlook.com and Yahoo, so you can read placement and headers yourself.
• People who asked to hear from you: anyone who signed up, replied on LinkedIn or met you at an event.
• A first small prospect batch: the most clearly qualified names on your list, written individually.

Write these as emails worth answering. Our guides on introducing yourself in an email and outreach cover the writing; warmup only cares that a human replied.

Email warmup tools and warmup networks

A warmup tool connects your mailbox to a pool of other mailboxes. It sends emails between them, opens them, replies, and pulls emails out of the spam folder.

Most sales engagement and cold outreach platforms bundle a warmup tool, and several vendors sell the tool on its own. The mechanics are similar everywhere: emails go from your mailbox to other inboxes in the network, get marked as important, collect a few replies, and get rescued from spam.

What an email warmup tool actually does

Strip the marketing away and every email warmup tool does four things. It sends emails from your mailbox on a schedule, opens them in the receiving inbox, replies to some of them, and moves any that landed in spam back to the inbox.

The fifth thing it does is report. The dashboard shows emails sent, emails that reached the inbox, emails rescued from spam, and an inbox placement percentage measured across the network of participating inboxes.

Notice that none of those questions are about price or feature count. They are about whether the tool gives you a deliverability signal you can act on, or a number that only describes life inside its own network.

One more distinction is worth knowing. A warmup network is the pool of inboxes your mailbox exchanges emails with. Its size is the number on the marketing page. Its composition decides whether the inbox placement figure means anything at all for your email deliverability.

Some warmup tools also warm a custom tracking domain and check your sending domain against blocklists. Those are useful checks, and they are also checks you can run yourself in a few minutes each week.

Bundled warmup or a standalone tool

Most cold email platforms include warmup, and standalone email warmup tools exist for teams whose sending platform has none. Bundled warmup is simpler because inbox rotation, daily caps and campaign sending all sit in one place. A standalone tool earns its place when you send from several platforms.

Before you commit to an email warmup tool

Open the vendor's own pages and read three things: how the network is built, which providers are represented in it, and what the free plan actually includes. Then check whether the features you need, such as inbox rotation and a daily cap per account, live in the warmup tool or in the sending platform.

Then run one mailbox through it for two weeks. If the inbox placement number never moves, and the warmup emails in your own inbox read like machine output, the tool is measuring itself rather than your email deliverability.

Free email warmup tools and what free usually means

Free warmup shows up in two forms. Some sending platforms include warmup for every mailbox at no extra cost, and some standalone tools offer a free tier with a small number of inboxes. In both cases the pool of inboxes is the product, and a free pool is usually the least curated one.

Paid tools are normally priced per mailbox per month, which is why teams running many inboxes end up weighing the cost of warmup against the cost of more mailboxes. This page carries no pricing and no vendor ranking, because both change faster than any guide can track.

Here is the honest part. Artificial engagement is not real engagement. A tool that opens and replies to its own network's emails is producing a pattern, and none of the three providers list warmup networks anywhere in their sender guidelines as something that earns trust.

What providers do say they measure is recipient reaction: user-reported spam, unsubscribe behavior, whether you send to people who opted in. A pool of cooperating mailboxes does not generate any of those signals from your actual audience.

• What a tool can genuinely do: keep a schedule you would otherwise forget, and give you a daily view of where test mail landed.
• What it cannot do: make a bad list good, fix a failing DKIM record, or offset complaints from people who did not want your mail.
• The risk nobody advertises: the quality of a peer pool is outside your control, and your mailbox sits in it alongside senders you cannot see.
• Treat the numbers carefully: a warmup dashboard showing high inbox placement is measuring mail inside the pool, not mail to your prospects.

Use an email warmup tool if it helps you keep a rhythm. Do not treat its dashboard as evidence that your campaign will land.

Inbox placement, the spam folder and what deliverability means

Deliverability gets used for two different things, and mixing them up is how senders convince themselves a broken setup works. One meaning is acceptance: the receiving server took the email. The other is inbox placement: the email reached the inbox instead of the spam folder.

A new domain can score perfectly on the first and badly on the second. Every email is accepted, every email is filed as spam, and your sending tool reports a delivery rate near 100 percent. That figure is the most misleading number in cold email.

Warmup influences inbox placement, not acceptance. So the check that matters is not the delivery rate on a dashboard. It is where your test emails actually landed in inboxes you can open and read yourself.

• Authentication: emails that fail SPF, DKIM or DMARC can be rejected or filed as spam whatever your reputation is.
• List quality: emails to addresses that no longer exist produce bounces, and bounces damage domain reputation fast.
• Recipient engagement: replies and rescues from the spam folder count for you; complaints and deletions without opening count against you.
• Volume pattern: a steady rate reads as a business, a spike reads as a bought list.
• Content and links: a cold email with five links, heavy images and a tracking pixel looks like marketing, because it is.
• Domain and IP reputation: the rolling record both accumulate, which is exactly what warmup is trying to start.

Keep a few inboxes you control at Gmail, Outlook.com and Yahoo, and send each campaign email to them alongside the campaign. Reading placement in real inboxes beats any deliverability score a tool prints for you.

Email deliverability checks to run every week

Checks turn warmup from a waiting game into something you can steer. Run the same short list weekly, at the same point in the week, so the results compare.

Two SMTP codes are worth recognizing. Gmail typically rejects with 4.7.28 when you exceed a sending quota, and its guidance is to stop sending for at least ten minutes before reconnecting with a single connection.

Outlook.com rejects mail from non-compliant high volume domains with 550 5.7.515 and a message saying the sending domain does not meet the required authentication level, according to Microsoft's announcement of the requirements.

What Google Postmaster Tools will and will not show you

Postmaster Tools is the closest thing to an objective scoreboard for Gmail, and it has two limits that matter during warmup.

First, the data covers only messages sent to personal Gmail accounts, those ending in gmail.com or googlemail.com. Business inboxes hosted on Google Workspace are not in the dashboards.

Second, Google states that data may be missing when the total number of messages for a given day is too low, to protect user privacy. A mailbox sending twenty emails a day will show empty dashboards, which is not a good sign or a bad one.

Set it up anyway, on the DKIM or SPF domain you authenticate with, so the history exists by the time your volume makes it readable.

How to warm up a new email domain and mailbox

1. Register the sending domain and point it somewhere real

   Buy a separate domain for outreach, redirect it to your website or give it a simple page, and keep your main domain out of cold campaigns.

2. Publish SPF, DKIM, DMARC and MX records

   Add all four before the mailbox sends anything. Set DMARC to p=none at first with a reporting address, so you can see what passes and what does not.

3. Verify authentication with your own test accounts

   Send to an address you control at Gmail, Outlook.com and Yahoo, open the raw headers, and confirm SPF, DKIM and DMARC all pass before anything else happens.

4. Finish the mailbox: name, signature, replies

   Set a person's display name, a plain signature, a From: address that accepts replies, and connect the mailbox to a client you actually read.

5. Spend the first weeks on people who answer

   Colleagues, partners, past contacts and your own accounts. Ask for real replies. Keep daily volume low and the rate consistent rather than bursty.

6. Increase volume gradually, never by doubling

   Add a little each few days. If bounces or deferrals rise, drop back until the error rate falls, then climb again from there.

7. Run the weekly checks and write the numbers down

   Authentication, placement, bounces, SMTP codes, blocklists and Postmaster Tools once it has data. A log turns a vague feeling into a trend.

8. Start campaigns with your best segment, small

   Send to the most clearly qualified prospects first, watch complaints and replies for a week, and only then widen the list.

Warning signs that mean stop, not push harder

The instinct when results dip is to send more to make the numbers back up. That is the one reaction that reliably makes things worse, because volume is the variable providers watch most closely.

What warmup does not end

Reputation is a rolling assessment, not a certificate. Providers keep judging recent behavior, so the habits from warmup become the habits of the program.

• Keep the volume cap: pick a daily ceiling per mailbox and add mailboxes instead of pushing one past it.
• Keep sending consistently: Google asks for a consistent rate and no bursts, which means Monday spikes are worth avoiding.
• Keep some real conversation: replies from people who know you do not stop mattering when campaigns start.
• Keep the list clean: remove bounces and anyone who asks out, which Microsoft and Yahoo both call out directly.
• Keep monitoring: the weekly check takes fifteen minutes and catches problems while they are small.

Once the domain is steady, the work moves back to the message and the rhythm. Our sales cadence guide covers the sequence side, and the sales outreach hub covers the rest of the channel mix.

Scaling across mailboxes and domains

Teams that need to send at scale add mailboxes rather than raising one mailbox's daily count. Each new mailbox and each new domain starts with no history, so each one needs its own ramp.

1. Stagger the starts, so you are not warming six new domains in the same week.
2. Give every domain its own warmup plan, with its own SPF, DKIM, DMARC and MX records, and verify each one separately.
3. Keep a record of which mailbox sends which campaign, so a problem can be traced to a source.
4. Rotate sending across mailboxes rather than exhausting one, and keep each within its cap.
5. Retire a domain that has collected complaints instead of trying to rehabilitate it under load.

Yahoo's guidance adds a useful principle for anyone with mixed traffic: separate email types by IP or DKIM domain, and do not send bulk or marketing mail from the IPs that carry transactional and user mail. The same logic is why outreach belongs on its own domain.

Consent, opt-out and the legal floor

Warmup is a deliverability practice. It is not a permission slip. In the United States, the CAN-SPAM Act requires accurate From, To and Reply-To information, a subject line that reflects the message, a valid physical postal address, and a clear explanation of how to opt out.

The FTC also requires opt-out requests to be honored promptly, and the opt-out mechanism to keep working for at least thirty days after you send. Gmail and Yahoo both ask bulk senders to process unsubscribes within about two days, which is stricter than the law.

Other countries set higher bars for consent. If you send to the European Union, the United Kingdom or Canada, check the rules that apply there before you rely on a warm domain to carry cold mail.

What to track during warmup

Open rate is missing from that list on purpose. Between image proxies and privacy protection it is a weak signal, and during warmup, replies tell you far more than opens.

The checklist before your first cold email campaign

Run this list the day before the first campaign. If any line fails, the campaign waits. A warm domain with one broken record still sends emails into the spam folder, and a warm mailbox with a bad list still collects complaints.

• SPF, DKIM and DMARC pass on test emails at each of the three big providers, read from the raw headers rather than from a checker tool.
• MX records resolve and a reply sent from an outside account lands in the inbox you actually read each day.
• The list is verified so your first cold outreach emails do not bounce on addresses that stopped existing months ago.
• The daily cap is written down per mailbox, and the sending tool enforces it instead of trusting you to remember.
• An opt-out route exists in every email, and a named person honors removals within two days.
• Postmaster Tools is connected for the authenticated domain, even while it is too quiet to show data.
• Warm traffic continues next to the campaign, so the domain keeps real conversations in its mix.
• Someone owns deliverability week to week, and has the authority to pause the campaign without asking.

Cold email deliverability is mostly this list plus patience. Your domain does not need to be famous, it needs to be boring and consistent, and your emails need to reach inboxes belonging to people who might actually want them.

Common email warm up mistakes

• Starting the warmup before SPF, DKIM and DMARC pass, so the whole ramp is built on a failing setup.
• Running cold campaigns from the main company domain and discovering the cost after support mail starts landing in spam.
• Treating a warmup tool's dashboard as proof of deliverability to actual prospects.
• Doubling volume week over week, which Gmail specifically warns can trigger rate limiting or a reputation drop.
• Warming up to an audience that cannot reply, so no real engagement signal is ever created.
• Ignoring bounces and deferrals instead of reading the SMTP codes they come with.
• Expecting Postmaster Tools to show data at warmup volumes, then concluding something is broken.
• Stopping all monitoring the day the first campaign goes out.
• Buying a list and using warmup to try to survive it.

The first message from a new domain

This is the kind of message to send in week one: short, from a person, to someone who will reply, with a reason to write that is not the warmup itself. Adapt it and send it by hand.

For the campaign messages that follow, our follow-up templates and the email marketing campaigns guide take over from here.

Subject: new address, quick favor

Hi {{firstName}},

I moved my outbound mail to {{newDomain}} this week, and I am sending the first few messages by hand so the address has a real history rather than a machine-made one.

If you have thirty seconds: hit reply with anything at all, even one word. It helps more than it sounds like it should.

While I have you, {{realReason}}.

{{senderName}}
{{role}}, {{company}}

Frequently asked questions