Cold email infrastructure playbook

Why cold email needs its own infrastructure

Cold email reaches people who haven’t heard from you before. Even when it’s relevant and lawful, it gets fewer replies and more “ignore” or “report spam” actions than email to customers. Those signals attach to the sending domain and mailbox.

If you send cold email from your company’s main domain, the same reputation also carries your invoices, password resets, support replies and the CEO’s email. One bad campaign can push all of it toward spam. Separate infrastructure isolates that risk.

The building blocks

Choosing secondary domains

Pick domains that are clearly related to your brand, so recipients recognise you and the domain doesn’t look like a disguise:

• getacme.com, acmehq.com, tryacme.io, acme-team.com
• Stick to common top-level domains. Some cheap, unusual TLDs carry poor reputations because of past abuse.
• Avoid hyphens and numbers when you can, and anything that could be confused with another company.

Domain age

A brand-new domain has no reputation at all, and some filters treat very young domains with extra suspicion. That’s a reason to buy secondary domains a few weeks before you need them and to warm them fully; it isn’t a reason to buy expired domains with unknown histories. A previously used domain may already be on blocklists or carry complaints. If you do buy one, run a blacklist check first.

Point the website somewhere

Set up a redirect from each secondary domain’s website to your main site. Recipients who type in the domain, and filters that look for a real web presence, both find something legitimate.

How many mailboxes per domain?

Two to three is the common practice. More mailboxes on one domain concentrate more volume and more risk on that domain’s reputation; fewer means buying and managing more domains.

How many mailboxes do you need?

Work backwards from your daily target:

The 30 campaign emails assume a mailbox at about 40 a day, keeping roughly a quarter for background warm-up. Follow-ups count toward the daily total, so a three-step sequence needs more capacity than its first step suggests. The warm-up calculator does this sizing for your numbers.

Which mailbox providers to use

Recipients use many providers, and each one judges senders slightly differently. A cold-email setup usually runs on one or two business mailbox providers:

• Microsoft 365 and Google Workspace are the most common. Both are widely trusted, but it pays to spread mailboxes across both rather than depending on one.
• Zoho Mail and other business providers work well and cost less per mailbox.
• Your own SMTP servers give you full control, and full responsibility for IP reputation, PTR records and TLS.

Whatever you send from, test where mail lands across the providers your recipients use. A setup that lands well at one provider can still miss at another. An inbox placement test shows that per provider.

Authentication on every secondary domain

Each secondary domain needs its own complete setup before it sends a single email:

1. SPF: one record that includes your mailbox provider and your sending tool if it sends directly. Build it with the SPF generator.
2. DKIM: enabled in the mailbox provider with a 2048-bit key, and custom DKIM in the sending tool if it signs separately.
3. DMARC: at least p=none with a report address. Create it with the DMARC generator.
4. MX records: so the domain can receive replies and bounces.
5. Custom tracking domain: if your sending tool tracks opens or clicks, use a tracking domain on your own secondary domain rather than the tool’s shared one, which can carry other senders’ reputations. Many teams turn open tracking off entirely for cold email.

Run the domain health check on every domain before warm-up starts.

Set up each mailbox like a real person’s

• A real first and last name, and a matching profile photo.
• A plain signature with name, role and company. Skip big images and many links.
• A few genuine messages sent and received before automation starts.
• The same sending location and connection every day. A mailbox that appears from a new IP each day looks compromised.

Warm up before you send

Every new mailbox needs two to four weeks of warm-up before campaigns, starting at 2-5 emails a day and growing to its working volume. Read the complete guide to email warm-up for the full method. Two rules matter most:

• Don’t send campaigns during the ramp. You’d spend reputation before it exists.
• Don’t stop warm-up when campaigns start. Keep 20-30% of volume as background warm-up for as long as the mailbox sends.

Sending limits per mailbox

Mail providers publish technical sending limits, but those are ceilings for normal business use, far above what a cold-email mailbox should send. For cold outreach:

• Stay at or below 30-50 emails per mailbox per day, including warm-up.
• Spread sends across the working day rather than in bursts.
• Keep follow-ups inside the daily cap.
• Increase volume gradually when you add a new campaign, even on warmed mailboxes.

Provider limits change, so check your provider’s current documentation, and see our provider guides such as warming up Outlook and Microsoft 365.

Inbox rotation

Rotation means spreading a campaign across many mailboxes so each sends only its share, and swapping mailboxes in and out as their health changes. Most sending tools do the spreading automatically when you assign several mailboxes to a campaign. The harder part is knowing when to rotate a mailbox out:

Warm replacement mailboxes before you need them, so a paused mailbox doesn’t leave a gap in the campaign.

List quality is infrastructure too

The best-configured mailbox can’t survive a bad list. Verify addresses before sending to keep hard bounces low, remove role addresses where they’re unlikely to welcome outreach, and never buy lists that were collected without a lawful basis. Bounces and spam-trap hits damage reputation faster than any warm-up can repair it.

Common infrastructure mistakes

• Using the primary domain “just for a test”. One test campaign can be enough to damage it.
• Too many mailboxes on one domain. Ten mailboxes on one secondary domain concentrate all of their risk on that domain.
• Skipping DMARC on secondary domains. Each domain is judged on its own; the primary domain’s record doesn’t cover a separate domain.
• Buying cheap mailboxes from unknown resellers. Shared or previously abused infrastructure brings someone else’s reputation with it.
• Turning off warm-up once campaigns start. Reputation fades within weeks without steady positive engagement.
• Watching open rates instead of placement. By the time open rates fall, the damage is done.

The setup checklist

1. Buy 2-3 secondary domains per planned group of mailboxes, a few weeks ahead.
2. Redirect each domain’s website to your main site.
3. Publish SPF, DKIM, DMARC and MX for every domain and verify them.
4. Create 2-3 mailboxes per domain with real names, photos and signatures.
5. Connect every mailbox to warm-up and let it ramp for 2-4 weeks.
6. Run a placement test before the first campaign.
7. Connect the mailboxes to your sending tool with sensible daily caps.
8. Monitor health, placement and blocklists, and rotate mailboxes out early.

RoxInbox runs steps 5, 6 and 8 for every mailbox: adaptive warm-up on every major provider, daily placement, and monitoring with alerts that pause a mailbox before it burns. Agencies running this for many clients can use client workspaces and bulk connect.

Written by the RoxInbox team

The people who build and run the RoxInbox warm-up network, placement tests and monitoring. We write from what we see across mailbox providers every day. About the authors