Cold email domain setup: domains, mailboxes and warmup

Updated October 25, 2026

Cold email domain setup means sending outreach from separate domains that resemble your main one, putting a few mailboxes on each, authenticating every domain with SPF, DKIM and DMARC, and warming each mailbox up before it sends campaigns. Done this way, a deliverability problem stays contained to a domain you can replace.

About the numbers on this page. Inboxes per domain, warmup length and daily send counts are practitioner benchmarks published by Instantly, a cold email platform. They are not limits set by Gmail or Outlook. Treat them as starting points and adjust to your own bounce, reply and complaint data.

Why send cold email from a secondary domain?

So that complaints, blocks and spam-folder placement caused by outreach land on a domain you can replace, not the one your team, invoices and customers rely on. Receivers track reputation per sending domain, and cold email carries more risk of complaints than mail people asked for.

Instantly describes secondary domains as a way to "protect your primary domain and let you scale sending volume." (Instantly)

How should I choose cold email domains?

Pick names a recipient would recognize as yours, such as your brand with a short prefix or suffix, on a common top-level domain. Instantly's advice is to "get domains similar to your primary one." Avoid names that imitate another company's brand. Register each domain, then point its website to your main site so curious recipients find you.

Separate domains do not forward replies to your main inbox on their own. Instantly recommends managing replies in each mailbox or in your outreach platform rather than setting up forwarding. (Instantly)

How many inboxes per domain?

About three. Instantly's guidance is two to three inboxes per domain, and it calls three sending accounts per domain best practice. Its done-for-you setup allows a maximum of five per domain. To send more, add domains rather than piling more inboxes onto one. (Practitioner benchmark: Instantly, Instantly, Instantly Help Center)

How long should I warm up new inboxes?

At least two weeks, and three to four weeks is safer for a brand new domain. Instantly's help center says to warm up accounts for at least two weeks before launching campaigns, and its blog says to plan for three to four weeks with a hard minimum of two.

Instantly's 2026 benchmark report puts the full ramp to stable volume at four to six weeks. Warmup starts small: Instantly suggests 5 to 10 warmup emails a day per inbox in the first week, rising toward 30 to 50 in later weeks. (Practitioner benchmark: Instantly Help Center, Instantly)

How many cold emails per inbox per day?

Practitioner benchmarks put it at roughly 30 campaign emails per inbox per day. Instantly's warmup guide says to keep campaign sends at or below 30 per inbox per day after launch; its infrastructure guide puts a fully warmed inbox at 40 to 50 emails a day in total, with warmup emails counting toward that number. (Instantly, Instantly)

These are far below the hard limits providers set. Google Workspace, for example, allows 2,000 messages a day per user. (Google Workspace) The lower practitioner numbers exist to keep complaint rates low and sending patterns close to a normal person's, not to stay under a provider cap.

How many domains do I need?

Divide your daily send target by sends per inbox, then by inboxes per domain, and round up. For example, at 30 campaign emails per inbox and three inboxes per domain, 300 emails a day needs 10 inboxes across 4 domains. Add a spare domain or two that are warming while the others send.

Example domain calculation
Step	Example
Daily target	300 emails
Per inbox (benchmark)	30 a day
Inboxes needed	300 / 30 = 10
Domains (3 inboxes each)	10 / 3 = 3.3, so 4

This is arithmetic on the benchmarks above, not a recommendation for any particular volume.

How do I authenticate cold email domains?

Give every sending domain its own SPF, DKIM and DMARC records, plus MX records so replies arrive. Each domain is checked on its own, so records on your main domain do not cover the others. Set up DKIM in your mailbox provider and in your sending tool so mail is signed with the sending domain itself.

SPF: one record per domain. For Google Workspace mailboxes, v=spf1 include:_spf.google.com ~all; for Microsoft 365, v=spf1 include:spf.protection.outlook.com -all. (Google, Microsoft)
DKIM: 2048-bit where supported, at least 1024 bits for Gmail. (Google)
DMARC: at least p=none with a rua address. Gmail, Yahoo and Outlook.com require this from senders above 5,000 messages a day. (Google, Yahoo, Microsoft)
MX: real mailbox MX records, so replies and bounces reach you.

Run each domain through the checker before warmup starts. The full walk-through is in the SPF, DKIM and DMARC setup guide.

What else affects cold email deliverability?

Complaints matter most. Google and Yahoo both ask senders to keep spam rates below 0.3%, and Google recommends staying below 0.1%. Beyond that: send to verified addresses so bounces stay low, make it easy to opt out and honor it quickly, and keep volume steady instead of sending in bursts.

Yahoo asks bulk senders to honor unsubscribes within 2 days. (Google, Yahoo)

Setting up new domains? Check each one's SPF, DKIM and DMARC before you start warmup.

Frequently asked questions

Can I use a subdomain instead of a separate domain for cold email?

You can, but a subdomain shares its organizational domain with your main brand. DMARC policy and reporting roll up to the parent, and a complaint problem is harder to isolate. A separate domain keeps outreach reputation apart from the domain your team and customers depend on.

Does warmup replace SPF, DKIM and DMARC?

No. Warmup builds sending history on an inbox, but receivers still check authentication on every message. Publish SPF, DKIM and DMARC on each domain before warmup starts, so the history being built is for authenticated mail.

How many emails can a Google Workspace inbox send per day?

Google's hard limit is 2,000 messages a day per user (500 for trial accounts). That is a ceiling for account abuse, not a cold email target: practitioner benchmarks for cold outreach are a few dozen a day per inbox. (Google Workspace)

Should cold email domains have a website?

It helps. A recipient who checks the domain should land somewhere that clearly belongs to you, which is why many senders redirect the secondary domain to their main site. This is common practice rather than a provider requirement.

Do I need DMARC on every cold email domain?

Yes. Each domain is checked on its own, and Gmail, Yahoo and Outlook.com require DMARC with at least p=none from senders above 5,000 messages a day. Across many inboxes, totals add up quickly, so treat every sending domain as a bulk sender. (Google, Microsoft)