Cold email infrastructure: domains, mailboxes and rotation

Cold email infrastructure is the domain, the mailboxes, their authentication and the schedule that sends from them. Use a secondary domain so your main one is protected, authenticate with SPF, DKIM and DMARC, start each mailbox at 5 to 10 emails a day, raise slowly and keep bounces below 2%. Add mailboxes for volume instead of pushing one past its limit.

There is a lot of folklore about cold email infrastructure: ten domains, thirty mailboxes, magic rotation. Most of it is a way to sell you more domains. This guide sticks to what the mailbox providers say, what a benchmark report found and what we ship, and it says plainly what none of them can promise. A well built setup lowers the chance that your mail lands in spam. It does not guarantee anything, and it cannot rescue a bad list.

What is the infrastructure made of?

Four parts.

1. A sending domain. Often a secondary domain that sits beside your main one, for example a variation of the company name.
2. Mailboxes on that domain. Real accounts on Google Workspace, Microsoft 365 or another IMAP host.
3. Authentication. SPF, DKIM and DMARC records that tell receivers your mail is genuine.
4. A schedule. Daily limits, send hours and a ramp that grows over weeks.

Our post on the minimum cold email setup explains the records one by one. Here we deal with the choices around them.

Should you use a separate domain?

Instantly's 2026 benchmark report warns that concentrating all volume on a single domain creates risk. The reasoning is simple. Mailbox providers score the sender's domain and address. If a cold campaign earns complaints, a bad reputation attaches to whatever domain sent it. If that domain is the one your invoices and customer replies come from, you have put the wrong thing at risk.

A secondary domain contains the damage. The trade off is that a new domain has no history, so it needs a ramp. Some points to hold on to:

• Keep the secondary domain close enough to your brand that a recipient who checks recognises you.
• Put a real website or a redirect to your main site on it. A bare domain looks like a throwaway.
• Do not buy a dozen look alike domains. It reads as evasion, and it makes every domain harder to manage.

How many mailboxes do you need?

Plan by daily volume, not by superstition. Sellinger's mailbox settings recommend 50 to 150 sends a day, and a mailbox created through Sellinger defaults to 50. Providers set their own sending rules on top, and a new mailbox sends far less in its first weeks.

The table is arithmetic, not advice to push each mailbox to its limit. Most teams stay near the low end. If you need more reach than a few mailboxes can carry, widen the list slowly instead of raising the cap.

Also decide who owns each mailbox. In an agent run campaign the mailbox belongs to a real sender, such as a founder, and replies come to that person. The Green Shield Logistics campaign wrote as its founder from the company's own mailbox. See the case study.

How do you warm up a mailbox?

Instantly recommends starting slow with 5 to 10 emails a day and increasing gradually over four to six weeks. Sellinger's optional warm up for email starts at 5 a day and doubles every three days up to the limit. Doubling every three days reaches 40 on day 9 and the 50 a day limit by about day 12, which is faster than Instantly's advice, so on a brand new domain extend the ramp by hand.

During a warm up:

• Send to the safest part of your list first, the people most likely to reply.
• Keep volume steady from day to day. Instantly notes that erratic volume hurts deliverability.
• Read bounces daily. Pause if they rise.

What does rotation do?

Rotation spreads one campaign over several mailboxes so each sends a modest number a day. It has real uses: lower volume per address, a backup if one mailbox is disabled and a natural split by sender. It also has limits. It does not hide a bad list from receivers, because complaints and bounces still count against the domain. And a recipient who gets emails from three of your addresses in a week will notice. Keep a rule of one sender per person: a contact is assigned to one mailbox and stays there for the whole sequence, so the thread is consistent.

What do Google and Yahoo ask?

Both ask senders to authenticate with SPF and DKIM, publish a DMARC policy, offer one click unsubscribe and keep the spam rate below 0.30%. Google adds that senders should aim for 0.10%. The formal bulk sender definition at Google is 5,000 or more messages a day, but providers score every sender on its behaviour, so small senders should follow the same habits. Our explainer on the Google and Yahoo rules has the details and the sources.

What should you monitor?

What does Sellinger handle, and what does it not?

Sellinger connects a Gmail, Outlook or IMAP mailbox, sets a daily limit per mailbox, follows a send window you choose, offers an optional warm up, can add an unsubscribe line and the standard one click unsubscribe headers, and checks the suppression list before every send. See email sending limits and daily limits and pacing.

It does not sell domains, build your DNS records for you or promise inbox placement. Those depend on your domain, your list and the recipients' behaviour. We make no deliverability claim, and you should be sceptical of anyone who does.

What mistakes cost the most?

• Sending from the main domain "just for now". The test that turns into the campaign is how reputation problems begin.
• Raising limits when replies are low. Low replies mean a list or message problem. More volume of the same only adds complaints.
• Skipping list checks. A bounce rate above 2% is a signal to stop and clean, not to keep going. Instantly says to pause the campaign and clean the list before continuing.
• Forgetting the schedule. Sending at 3 a.m. in the recipient's time zone looks automated. Choose days, hours and a time zone per mailbox. See send windows and time zones.
• Changing everything at once. If you move to a new domain, a new message and a new list in the same week, you cannot tell what caused a change in results.
• Ignoring replies. Mailbox providers watch engagement. A mailbox that nobody answers from is a bad sign, so route replies to a person or an agent that answers in minutes.

A launch checklist

• [ ] A secondary domain with a real website or redirect.
• [ ] SPF, DKIM and DMARC pass for every mailbox.
• [ ] Each mailbox starts at 5 to 10 a day and ramps over weeks.
• [ ] The list is verified and bounces are watched daily.
• [ ] One sender per contact for the whole sequence.
• [ ] An unsubscribe mechanism works, and opt outs are honored.
• [ ] I know the spam rate from Postmaster Tools.