Divide your daily send target by 30, the number of cold emails one mailbox can safely send in a day, and you have the inboxes you need. Divide the inboxes by 3, the most we put on one domain, and you have the domains. 300 sends a day is about 10 inboxes on 4 domains. 1,000 a day is about 33 inboxes across 11 to 12 domains. None of them is your primary domain.
This is for founders and SDR leads setting up cold email for the first time, or scaling past the two mailboxes they started with. It covers the sizing formula, the three-inbox ceiling, naming, what the sending layer costs each month, and when to rest or retire a domain. The rest of the stack, from data to sequencer to CRM to reporting, is in our guide to outbound sales infrastructure.
How do you calculate the domains and inboxes you need for cold email?
Two divisions. Daily sends divided by 30 gives inboxes; inboxes divided by 3 gives domains. Round up and add a spare. The 30 is the safe ceiling of 20 to 40 sends per mailbox per day on a warmed domain. The 3 is how much volume we are willing to put behind a single domain.
| Daily sends | Inboxes (30 a day each) | Domains (3 inboxes each) | New prospects a month |
|---|---|---|---|
| 150 | 5 | 2 | About 700 |
| 300 | 10 | 4 | About 1,500 |
| 600 | 20 | 7 | About 2,900 |
| 1,000 | 33 | 11 to 12 | About 4,900 |
The last column is the one people skip. At 4 to 5 emails per prospect, 1,000 sends a day consumes about 4,900 new, well-fitted prospects every month. Few companies selling above $10,000 a year can find that many, and they do not need to: a tight list books a meeting for every 100 to 150 emails, a generic one needs 800 or more. Most programmes we run sit between 300 and 600 sends a day. The per-mailbox ceiling itself is explained in how many cold emails you can send per day.
Why not put more than three inboxes on one domain?
Because reputation is scored per domain as well as per mailbox. Three inboxes at 30 a day put about 90 sends behind one domain, which is what a small company looks like. Ten inboxes put 300 a day behind it, which is what a bulk sender looks like, and one complaint on one mailbox then damages all ten.
Spreading volume also isolates damage. Lose one domain in a 33-inbox setup and you lose 3 inboxes, under 10% of capacity, while the campaign keeps running. Lose a domain carrying 10 inboxes and a third of your volume stops that afternoon. Three is a ceiling, not a target; some teams run two per domain for extra margin at the cost of a few more domains to warm. Rotating sends evenly across mailboxes, and the records that let a domain send at all, are covered in inbox rotation and SPF, DKIM, DMARC.
What should you name your secondary domains?
Buy variants of your brand that a prospect would recognise: names in the style of trymarginsales.com or marginsaleshq.com, never marginsales.com itself. Cold email does not go out from your primary domain. A burned secondary domain costs a few weeks and a few dollars. A burned primary domain sends your invoices, support replies, and founders' mail to spam along with it.
- Readable and honest. Someone who sees the domain in a From line should be able to guess who you are. Prefixes like try, get, and hello, or suffixes like hq and team, work. Numbers, hyphens, and strings of initials look like spam.
- Mainstream endings. A .com first, then .co, .io, or .net. Unusual endings filter worse and look like phishing to a cautious buyer.
- Your brand only. Never a variant of a competitor's or a customer's name, however tempting the open rate.
- Redirect every domain to your real site. A prospect who types it should land on you, and a domain with a live site behind it reads as a business rather than a throwaway.
- Real names on the mailboxes. firstname@ for a person who reads the replies, not sales@ or info@. Replies are the whole point, and a reply to a role address gets missed.
- Keep a register. Domain, mailboxes, campaign, the dates it was bought, warmed, and went live, and its current status. At a dozen domains you will not remember which one was rested last month.
What does the cold email sending layer cost per month?
Roughly $150 to $400 a month for 1,000 sends a day, directional, covering 11 to 12 domains, about 33 mailboxes, and warm-up and placement monitoring. Prices differ by provider and change often, so read the table as a shape, not a quote. The sending layer is the cheapest part of outbound; data and people cost several times more.
| Component | Quantity at 1,000 a day | Directional monthly cost | Notes |
|---|---|---|---|
| Domains | 11 to 12, plus 1 to 2 spares | $10 to $20 | Annual registration spread over the year; .com where available |
| Mailboxes | About 33 | $100 to $260 | Mainstream providers cost more per seat than cold email inbox providers; we prefer mainstream for reputation |
| Warm-up and placement tests | All mailboxes | $30 to $100 | Often bundled with the sequencer |
| Total sending layer | $150 to $400 | Excludes sequencer, data, and the people sending |
At 300 sends a day, plan on about a third of that. For scale, data alone runs $500 to $1,500 a month per rep, and a fully loaded SDR costs 1.7x to 2.5x base salary. If a vendor quotes a large 'infrastructure' fee, ask what is in it. The domains and inboxes are not where the money goes.
Sizing and building this layer is the first thing we do in every programme, and the domains are bought in the client's name. If you would rather have it built than build it, that is part of our outreach service.
When should you rest or retire a sending domain?
Rest a domain when its numbers drift from its siblings: bounces above 2% to 3% on a verified list, a reply rate falling while the other domains hold, a spam rate at or above 0.1% in Postmaster Tools, or a failed placement test. Rest means two to four weeks at warm-up volume only. Retire it if the rest does not bring it back.
Warm-up cannot be rushed: week 1 at 5 to 10 a day, week 2 at 10 to 20, week 3 at 20 to 30, week 4 at 30 to 40, and never real volume before two weeks. The schedule is in how to warm up a new email domain. Because of that lead time, keep one or two spares warmed and idle, so a retired domain is swapped rather than mourned.
Do not try to fix a burned domain by rewriting the copy. The address is the problem, not the words. Keep the registration and the redirect so nothing points at a dead domain, and do not reuse it for months; some never come back. The warning signs that appear before a blacklist does are in how to keep your domains off spam blacklists.
When do you not need secondary domains at all?
When you are not sending cold email. Follow-up to inbound leads, replies to people who wrote first, and mail to customers belong on your primary domain, and moving them would look strange to the recipient. Secondary domains exist for one job: unsolicited first contact at volume.
Two edges. A founder sending 10 to 15 hand-written emails a day needs one secondary domain with one mailbox, warmed properly, and nothing more. At the other end, if you cannot fill 300 sends a day with prospects who fit, do not build for 1,000. Sending more to a worse list moves you from 100 to 150 emails per meeting towards 800 or more, whatever the domain count. Get authentication, list hygiene, and monitoring right first with the cold email deliverability checklist.
How MarginSales approaches domains and inboxes
MarginSales provides sales outreach services for companies that want to extend their outbound capacity without building the entire sales development function internally. Domains are the first thing we buy in a new programme, on day one and in the client's name, because the 3 to 4 week warm-up is the longest lead time in the setup. We run 2 to 3 mailboxes per domain at 20 to 40 sends each, keep a register of every domain and its status, check Postmaster Tools weekly, and hold one or two warmed spares on the bench.
Warm-up, rotation, and placement tests are automated, because they are the simple part. The mailboxes carry the real names of the people on the pod, because a reply should land with a person who reads it and answers it the same day. That is the rule we apply across outreach: automate the simple, keep humans on the meaningful.
Frequently asked questions
How many email domains do I need for cold email?
Divide your daily send target by 30 to get inboxes, then divide the inboxes by 3 to get domains. 300 sends a day is about 10 inboxes on 4 domains. 600 a day is about 20 inboxes on 7 domains. 1,000 a day is about 33 inboxes across 11 to 12 domains. Add one or two spare domains, warmed and idle, and never use your primary domain.
Can I send cold email from my main company domain?
No. Cold email carries bounce and complaint risk however well you run it, and a burned primary domain sends your invoices, support replies, and founders' mail to spam with it. Buy variants of your brand (names in the style of trymarginsales.com rather than marginsales.com), redirect them to your site, and keep every cold send on those.
How many inboxes per domain is safe for cold email?
Two to three. At 30 sends a day each, three inboxes put about 90 sends a day behind one domain, which looks like a small company. Ten inboxes put 300 a day behind it, which looks like a bulk sender, and one complaint on one mailbox then damages all ten. Three is our ceiling, not a target.
How much does a cold email sending setup cost per month?
Directionally $150 to $400 a month for 1,000 sends a day: 11 to 12 domains at roughly $10 to $20 a month in spread annual fees, about 33 mailboxes at $100 to $260, and $30 to $100 for warm-up and placement monitoring. At 300 a day, plan on about a third of that. Data and people cost several times more than the sending layer.
Get your sending setup sized before you buy anything
Tell us your target daily volume, your deal size, and how many well-fitted prospects you can find a month. We will send back a sizing sheet: inboxes, domains, names to register, a warm-up calendar, and a directional monthly cost. Book the sizing call. Twenty minutes, and the sheet is yours whether or not we run the campaign.