Email Warmup: How to Warm Up a New Domain and Mailbox for Cold Email

A new domain and a new mailbox have no sending history. To Gmail, Outlook, and every other mailbox provider, that is not neutral. It is unknown, and an unknown sender that suddenly pushes hundreds of cold emails a day looks exactly like a spammer. Email warmup is the process of building that history slowly, so providers see a mailbox that behaves like a real person before it starts carrying campaign volume.

This vendor-neutral guide covers separate sending domains, SPF, DKIM, and DMARC, a week-by-week schedule, mailboxes per domain, the signals that show whether warmup is working, and when to stop sending. The numbers are common practice, not guarantees. No schedule can promise inbox placement, because the final judge is how real recipients respond.

What email warmup does, and what it does not

Mailbox providers decide where a message lands using many signals, and many are about the sender: domain and IP reputation, authentication, past volume, and how recipients reacted. A fresh mailbox has almost none of that history.

Warmup fills the gap gradually. There are two common approaches:

• Manual warmup. You send a small number of real emails each day to colleagues, partners, and contacts who will open and reply, and increase slowly.
• Automated warmup. A tool exchanges messages between participating mailboxes, opens them, replies, and moves them out of spam when needed, at a volume that rises over time.

Most teams combine automated warmup with a slow ramp of real sends. Either way, the goal is a steady, believable pattern of low-volume sending with positive engagement.

What warmup does not do matters just as much. It does not repair a bad list, cancel out spam complaints from real recipients, or make an aggressive volume plan safe. Warmup is one input to cold email deliverability, not the whole thing. If the campaign that follows targets the wrong people, the reputation you built gets spent in days. The AI SDR deliverability guide covers that upstream side: ICP rules, buyer signals, and channel choice.

Use separate cold email domains, not your main domain

Cold outreach carries reputation risk your main domain should never absorb. If a campaign goes wrong on yourcompany.com, invoices and your CEO's replies can start landing in spam too. A dedicated sending domain isolates that risk.

Practical rules for cold email domains:

• Pick close variants of your brand. Patterns like getyourcompany.com, yourcompanyhq.com, or tryyourcompany.com tell recipients who you are without touching the main domain.
• Prefer common top-level domains. A .com is usually safest. Avoid unusual TLDs, hyphens, and number strings, which read as throwaway domains.
• Point the domain's website to your main site. Visitors should land on your real company, not a parked page.
• Use real names on real mailboxes. A firstname@ address with a consistent display name and signature looks like a person. A sales1@ address does not.
• Buy more than one. Spreading volume across several domains means one problem domain slows you down instead of stopping you.

Keep newsletters and product email off your cold domains. Cold, marketing, and transactional email have different risk profiles and should not share a reputation.

SPF, DKIM, and DMARC for cold email

Authentication is no longer optional. The bulk sender rules Google and Yahoo began enforcing in 2024 require every sender to authenticate, and anyone sending more than 5,000 messages a day to their users needs SPF, DKIM, and DMARC together, with the From domain aligned. Microsoft added similar requirements for Outlook.com in 2025. Even far below those volumes, missing records are an easy reason to filter you.

Here is what each record does:

• SPF (Sender Policy Framework) is a TXT record that lists which servers are allowed to send mail for your domain. The receiver checks the sending server against that list. A domain should have exactly one SPF record, and it must stay under 10 DNS lookups or it fails.
• DKIM (DomainKeys Identified Mail) adds a cryptographic signature to every message. The public key lives in DNS at a selector such as selector._domainkey.yourdomain.com, and receivers use it to confirm the message was not altered and really came from your domain.
• DMARC (Domain-based Message Authentication, Reporting and Conformance) is a TXT record at _dmarc.yourdomain.com. It tells receivers what to do when a message fails SPF and DKIM alignment, and where to send reports. A common starting record is v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.com, which monitors without blocking. Once reports look clean, many teams move to p=quarantine.

Two other records matter: MX records so the mailbox can receive replies, and a custom tracking domain if you track opens or clicks, so your links do not share a generic domain with other senders.

To check your setup, send a test message to a Gmail address, open it, and choose "Show original." You want SPF, DKIM, and DMARC all showing PASS. Do this for every new domain before warmup starts, and again whenever you change DNS or providers.

How many mailboxes per domain

One mailbox concentrates risk, and stacking many mailboxes on one domain ties them all to the same domain reputation. The usual middle path is two to five mailboxes per sending domain, each with its own daily cap, spread across several domains.

Plan capacity backwards from the volume you need, and remember that follow-ups use sending capacity too. A sequence with a first email and three follow-ups can create four sends per prospect over a few weeks.

If you need more volume, add domains and mailboxes and warm them up, rather than raising per-mailbox caps on the ones you already have.

A week-by-week email warmup schedule

The schedule below reflects common practice for a brand-new domain and mailbox. Treat each row as a ceiling, not a target. Move forward only when the previous week looked healthy, and hold or step back when it did not.

A few rules make the schedule work:

• Increase in steps, not jumps. Raise volume every few days, and never double it from one day to the next.
• Keep weekends lighter. Real people send less on Saturday.
• Stagger mailbox starts. Start new mailboxes in waves so one setup mistake does not hit all of them.
• Put the best contacts first. Early cold sends go to verified addresses in your tightest segment.

How long to warm up an email domain

When you warm up a new domain from scratch, plan on three to four weeks before a mailbox carries normal cold volume. Two weeks is the usual minimum, and only when the first cold batches are small and the list is clean. Rushing is the most common reason new infrastructure burns out in its first month.

How long an email warm up takes also depends on:

• Domain age and history. A domain with months of normal mail behind it usually warms faster than one bought yesterday.
• List quality. Clean, verified, well-targeted lists earn replies that help. Bad data slows everything down.
• Past damage. A previously burned domain can take longer than a new one. Sometimes it is cheaper to retire it.

Warmup also does not truly end. Most teams leave it running at a low level for as long as the mailbox sends cold email.

Signs warmup is working, and signs it is failing

Check a few signals every day during the first month.

Healthy signs:

• Hard bounces stay well under 2% of sends.
• Warmup and test messages land in the primary inbox, not spam.
• SPF, DKIM, and DMARC keep passing on every message.
• Real recipients reply at a rate similar to your other healthy mailboxes.

Warning signs:

• Bounces climb after a volume increase or a new list source.
• Test messages start landing in spam or promotions.
• Authentication failures appear, often right after a DNS change.
• Receiving servers return deferrals ("try again later") or rejections.
• One mailbox falls behind the others on the same domain.

Treat open rates with care. Apple Mail Privacy Protection and similar features load tracking pixels automatically, so opens can look healthy while placement slips. Replies and bounces are more honest. The deliverability triage guide goes deeper on reading those signals once campaigns are running.

Keeping your reputation after warmup

Warmup earns you a reputation. Campaign behavior decides whether you keep it.

• Verify the list before it sends. Invalid addresses are the fastest way to turn a warmed mailbox into a bouncing one.
• Target tightly. People who fit your ICP and have a current reason to hear from you reply more and complain less.
• Send plain text. Short emails with few or no links look like personal mail. Heavy HTML, images, and attachments look like marketing.
• Cap volume per mailbox. Keep each mailbox at or below about 50 sends per day, follow-ups included, and scale by adding mailboxes.
• Rotate across mailboxes. No single warmed sender should carry the whole campaign.
• Space follow-ups. Two to four business days between steps is common. Stop the sequence when someone replies or asks to be removed.
• Be careful with tracking. If you track opens or clicks, use a custom tracking domain. Many teams turn tracking off for cold email entirely.
• Make opting out easy. A simple line such as "reply 'no' and I won't follow up" reduces spam complaints. Keep complaints well under 0.1%.

When to pause a mailbox or domain

Pausing early is cheaper than repairing a burned domain. Stop cold sending when you see:

• Hard bounces above roughly 3% on a batch, or a bounce cluster from one list source.
• Spam complaints drifting toward 0.1%.
• Any SPF, DKIM, or DMARC failure.
• Repeated deferrals or rejections from a major provider.
• A sudden drop in replies right after a volume increase.
• A listing on a major blocklist.

Stop cold volume first, then find the cause: list source, DNS change, volume jump, or copy. Fix it, restart at a lower step of the schedule, and climb again. If a domain keeps failing after a clean restart, retire it.

How warmup works in Reach Agents

Reach Agents keeps the infrastructure steps in one place, so the plan above is easier to run.

• Buy sending domains and mailboxes directly in Settings → Email Accounts. DNS, SPF, and DKIM are configured automatically, each domain supports up to 5 mailboxes, and domain renewals are handled. DMARC is not set up for you, so check that a DMARC record exists and add one if it does not.
• Or connect what you already have. Add your own SMTP/IMAP mailboxes one at a time or by CSV import, or import mailboxes from Zapmail.
• Automatic warmup. SMTP mailboxes are warmed up automatically by a background warmup job that runs every 30 minutes.
• Sending safeguards. Addresses are verified before sending, bounces and authentication failures are handled, and senders that need attention are flagged so problems surface before they spread.
• Volume control. Campaigns rotate across the mailboxes you select, each mailbox has a daily limit (50 by default), emails go out as plain text, and open and click tracking is optional, with a custom tracking domain.
• Sequences that stop on time. A campaign has a first email plus up to four follow-ups. Follow-ups stop on reply or suppression, and replies sync into one unified Inbox.

No tool can guarantee inbox placement, and Reach Agents does not claim to. What it does is make the conservative path the easy one.

Frequently asked questions

What is email warmup?

Email warmup is the practice of gradually increasing the volume sent from a new domain or mailbox so mailbox providers can build a positive sending history before cold campaigns begin. It usually combines automated warmup traffic with a slow ramp of real emails.

How long does it take to warm up a new email domain?

Plan on three to four weeks for a brand-new domain before a mailbox carries normal cold volume, with two weeks as a practical minimum. Older domains with some normal history often warm faster. Move to the next step of the schedule only when bounces, replies, and authentication results all look healthy.

Can I send cold email from my main company domain?

You can, but most teams should not. If your main domain gets filtered, everyday business email suffers too. Use separate, authenticated cold email domains that are close variants of your brand.

Should I keep warmup running after campaigns start?

Yes. Most teams keep warmup running at a low level for as long as a mailbox sends cold email, because it keeps a baseline of positive engagement under campaign traffic. Keep the per-mailbox daily cap in place.

How much does Reach Agents cost for cold email?

Email is priced by sending setup, so talk with us about the domains and mailboxes you need. Purchased mailboxes are billed monthly, and Reach Agents pricing has the details. The 7-day free trial includes 2 connected accounts and $15 in credits, and no card is needed to start.

Start your 7-day free trial at app.reachagents.ai: buy or connect a sending domain, confirm SPF, DKIM, and DMARC pass, let warmup run, and send your first small batch only when the mailbox is ready.