How many inboxes do I need for cold email?

The formula

That's the entire model. Every other number — bounce rate, warmup runway, sending window — feeds into one of those four lines.

Plug in the constants

• 30 sends per inbox per day — the deliverability cap. Push higher and providers flag you.
• 3 inboxes per domain — more inboxes on the same domain triggers spam pattern-matching.
• ~5 sending days per week — weekdays only; weekend sends underperform and look spammy.
• 5–7 touches per contact — most replies happen between touch 2 and touch 4.
• 3 weeks of warmup — required before any new inbox sends real cold mail.

Worked example: 1,000 contacts, 5 touches, 2-week window

1. Total sends = 1,000 × 5 = 5,000
2. Sending days = 14 calendar days × 5/7 = 10 weekdays
3. Sends per day = 5,000 ÷ 10 = 500/day
4. Inboxes = 500 ÷ 30 = 17 inboxes
5. Domains = 17 ÷ 3 = 6 domains

If your event is in 6 weeks, you need to start warming those 17 inboxes 3 weeks before the send window opens, which means provisioning the domains and inboxes this week.

Scenarios at a glance

All scenarios assume 5 touches per contact and a 2-week sending window. Stretch the window to 4 weeks and the inbox count roughly halves. Compress to 1 week and it doubles.

What changes the answer

Sending window length

The longer your window, the fewer inboxes you need. For the same 1,000-contact / 5-touch campaign:

• 1 week (5 sending days) → 34 inboxes
• 2 weeks (10 days) → 17 inboxes
• 4 weeks (20 days) → 9 inboxes
• 8 weeks (40 days) → 5 inboxes

Pick a window that matches your urgency. Event-driven campaigns (conferences, launches) usually need 2–3 weeks. Always-on outbound can spread across 4+ weeks per cohort.

Number of touches

More touches = more sends = more inboxes. A 7-touch sequence needs 40% more infrastructure than a 5-touch one. We rarely recommend more than 7 — reply rate gains plateau, and unsubscribe rates climb.

Sends per inbox per day

The 30/day rule is the safe default. You can push to 50/day on a well-warmed domain with high engagement, but every extra send per day raises the chance of getting flagged. Don't optimize this — optimize list quality and personalization instead.

Back-calculating from your event date

Most campaigns have a hard date — a conference, a product launch, a quarter-end. Work backward from that date:

1. Event date — when does the campaign need to land?
2. Minus 2 weeks — sending window starts
3. Minus 3 weeks — warmup begins (= 5 weeks before event)
4. Minus a few days — domain registration + DNS propagation + inbox setup

So if your event is 6 weeks out: provision domains and inboxes this week, start warming next week, begin real sends 3 weeks before the event.

If your event is less than 5 weeks out and you don't already have warmed inboxes, you have two options:

• Skip cold email for this campaign — use LinkedIn, paid, or warm intros
• Use a partner with existing warmed infrastructure (this is literally what we do at Allston Labs)

When NOT to set up your own infrastructure

Building cold email infrastructure in-house makes sense if:

• You're running outbound continuously (not one-off)
• You have someone on the team who'll babysit deliverability
• Your brand needs to come from your own domain (legal/IR-sensitive)

It usually doesn't make sense for:

• One-off event campaigns where you only need it for 4–6 weeks
• Early-stage teams where the founder is doing it themselves
• Anyone whose primary cost is the time to manage the stack

Use the calculator

Plug your real numbers into the calculator to get the exact inbox count, domain count, warmup timeline, and expected reply / meeting range for your specific campaign.