How many mailboxes do you actually need?
Work backwards from your pipeline goal to the exact infrastructure: domains, mailboxes, daily volume, and monthly cost — using safe 2026 sending limits.
The math behind safe cold email volume
Three mailboxes per domain is the convention for a reason — it keeps blast radius tight (one ban = one domain affected) and avoids the per-domain volume that makes Google flag a workspace as a sending farm.
Fifteen emails per mailbox per day sits comfortably below Google's and Microsoft's 2024+ bulk-sender flagging thresholds. The industry debate ranges from 8 to 30, but 15 is the safe default — and warmup ramps you there over 14–21 days starting at 5–8/day and adding 2–3/day.
Worked example: 20 meetings/month at 3% reply, 25% reply→meeting = 20 ÷ (0.03 × 0.25) = 2,667 emails. At 15/mailbox/day × 22 sending days = 330/mailbox/month. 2,667 ÷ 330 = 9 mailboxes across 3 domains. Cost: 9 × $2.50 + 3 × ($13/12) = $25.75/mo all-in.
Oversize slightly. Plan for +20% buffer for replacements, A/B sends, and rotation headroom. The cost of an extra domain is trivial; the cost of being capacity-constrained mid-campaign is missed pipeline.