How Many Cold Email Domains Do I Need for 300 Emails per Day? (Full Formula)

The Formula (Three Lines, No Magic)

inboxes  = daily_sends ÷ per_inbox_cap     (30 Google-targeted · 15-20 Microsoft-targeted)
domains  = inboxes ÷ inboxes_per_domain    (2-3, see the ratio guide)
reserve  = +20-30% capacity warming        (rotation + retirement buffer)

Every disagreement you've read about domain counts is a disagreement about the two input constants, not the arithmetic. We covered why 2-3 inboxes per domain is the right middle and why ~30/day per inbox keeps you out of bulk-pattern territory. Lock those and the answer stops being controversial.

Worked Examples: 100 / 300 / 500 / 1,000 per Day

• •100/day: 4 inboxes → 2 domains (3 with reserve). The starter footprint, one domain is possible but leaves zero redundancy.
• •300/day: 10-11 inboxes → 4-5 domains (5-6 with reserve). The most common serious-solo/small-team scale.
• •500/day: 17 inboxes → 6-8 domains (8-10 with reserve). At this scale rotation stops being optional, see below.
• •1,000/day: 34 inboxes → 12-17 domains (15-20 with reserve). Agency/fleet territory: per-client isolation, monitoring, and a real warming pipeline.

Split fleets adjust per segment: if your list segments 60% Google / 40% Microsoft (check yours with the ESP Segmenter), the Microsoft share runs at half the per-inbox cap, a 300/day mixed operation lands closer to 7-8 domains than 5.

The Buffer Everyone Forgets

Calculators that stop at "4-5 domains" assume nothing ever goes wrong. Real fleets rest inboxes on cycles, occasionally retire a domain, and never want a warmup delay standing between them and Monday's sends. That's the 20-30% reserve: capacity that's warming while the active pool sends, promoted when needed. The full active/resting/warming structure is in the rotation system guide, at 300/day and above, it's the difference between an incident and an outage.

Setup Order, Once You Have the Number

• •1. Buy believable brand-variant domains (never your main, why), 301 their websites to your real site.
• •2. 2-3 real users per domain, full SPF/DKIM/DMARC, verified with the Deliverability Checker.
• •3. Warm 2-4 weeks (or start prewarmed and skip to sending day one).
• •4. Ramp: 25% → 50% → 100% of target over 3 weeks, watching bounces against the stop rules.