Volume math: what 10k and 100k emails a day cost

Cold email scales horizontally. Each mailbox sends a little, so volume means many mailboxes on many domains. The per-mailbox number has fallen. Instantly's warmup guide, written in 2022 and still live, tells users to ramp a fresh domain "until you're sending 80-100 emails per day". Infrastructure vendors' 2026 pricing pages recommend 30, with a 100 maximum (Mailforge) and cap mailboxes at 25 on Google and 10 on Microsoft (ScaledMail) and 15 (Maildoso). A team sending 100 emails a day per mailbox in 2022 needs 4–10 times the mailboxes for the same volume in 2026.

That compression is the single most important number in cold-email economics. It drives how many mailboxes and domains you buy, how much you churn, and how visible your footprint is to Google and Microsoft. It also explains why "unlimited mailboxes" became the dominant sequencer pricing model (see Pricing landscape and Inbox rotation).

#Practitioner norms, October 2026

All prices are vendor list prices as of October 2026. Native Google Workspace and Microsoft 365 licences cost more than reseller mailboxes.

#The arithmetic

Assumptions, stated so you can change them:

• 20 sends per active mailbox per day (the middle of the 10–30 band)
• 30% extra mailboxes held in warmup or rest (my assumption; vendors don't publish a reserve ratio)
• 22 sending days a month
• Domains at $14/year
• No sequencer, data or verification costs (see Unit economics and Build costs and team)

Google Workspace mailboxes via a reseller ($3.50/mailbox, 3 per domain):

Microsoft via a reseller ($50/domain for 25 mailboxes at 10/day, i.e. 250 sends per domain):

SMTP/"private" mailboxes (Maildoso-style, 15/day, 4 per domain): 100,000 a day needs about 8,700 mailboxes on about 2,170 domains. That is roughly $4,250/month in mailboxes if Maildoso's $0.49 rate at 1,000 mailboxes held at that scale (unverified), plus about $2,530 in domains. Total around $6,800/month, roughly a quarter of the Google route. The discount is the market's price on the deliverability gap between Google/Microsoft IPs and SMTP IPs. See Google vs Microsoft vs SMTP and ESP matching.

ScaledMail's own worked example roughly matches. It quotes 2,000 a day split across Google, Microsoft and SMTP as 42 domains and 218 mailboxes for $397.75/month, plus $256 for optional reporting.

#The constraint nobody budgets for: contacts

Sends are not contacts. With a 4-step sequence (Instantly recommends 4–7 touches), each new prospect uses four sends:

At 100k/day you need half a million fresh, verified, in-ICP contacts every month. Most ICPs are exhausted within a quarter. Data, enrichment and verification then cost more than sending (see Contact data and enrichment and Email verification). Expected replies scale with whichever benchmark you believe. At 10,000 sends a day, Instantly's 3.43% average implies about 340 replies a day. Belkins' 0.45% implies about 45. See Reply benchmarks.

#Ramp schedules

No mailbox provider publishes a cold-email ramp. Google's general rule is the binding one: "immediately doubling previously sent volumes suddenly could result in rate limiting or reputation drops", plus "send email at a consistent rate. Avoid sending email in bursts." Practitioner practice:

1. Register domains and let them age, with warmup only, for 2–3 weeks. Instantly says to start at "just 2 emails per day" and climb "over 2-3 weeks (or longer)".
2. Start live campaigns at a fraction of the target per-mailbox volume and step up no faster than doubling.
3. Hold 20–30% of mailboxes in reserve so burned domains can be swapped without a fresh 2–3-week wait.

New Google Workspace tenants add a hard ramp: trial accounts are limited to 500 messages a day per user, and limits rise only after the domain has paid $100 cumulatively, which can take up to 75 days. Pre-warmed, pre-aged inventory is therefore something resellers can sell. See Does warmup work?.

#The footprint problem

At 100k/day on Google you are running about 6,500 Workspace users on about 2,200 domains. Each one shares patterns with the rest: registrar, DNS host, naming scheme, payment method, tracking domain, sending tool, send times and near-identical copy. Two policy facts make that footprint a liability:

• Google's Workspace AUP prohibits using the service to "generate, distribute, publish or facilitate unsolicited mass email" and "to resell End User Accounts, or parts thereof, as added into a commercial product offered to third parties". The second clause describes the reseller-mailbox business model. See Provider rules: Google, Microsoft and the ESPs.
• Gmail tracks volume, feedback and limits per domain and IP. Spreading volume across domains spreads quotas, but it does not hide the shared fingerprint from a provider that chooses to cluster.

#What this means for an entrant

• Price per mailbox is a race to zero, and per-send capacity is the real unit. Infra costs ~$11–12 per 1,000 sends on Google/Microsoft. A sequencer that charges by contact or by send captures value tied to outcomes. "Unlimited mailboxes" pricing just passes the infra bill through. See Pricing strategy.
• Automate the domain lifecycle. Buying, aging, warming, rotating, resting and retiring hundreds of domains is the hidden labour at 10k+/day. A platform that owns this lifecycle end to end (see Sending architecture) removes the most operational pain.
• Show customers the contact math before they buy volume. A planner that turns "10k/day" into "55k verified contacts a month and N meetings at your historical reply rate" sells better than raw capacity, and steers customers away from burning their TAM.
• Keep provider mix flexible. Google and Microsoft cost about the same per send. SMTP costs about a quarter of that and carries a different deliverability and policy risk. Don't hard-wire one. See Infrastructure strategy: build or partner.
• 100k/day is an agency and info-economy business, not an SMB one. The footprint, contact burn and AUP exposure make it a niche with high churn and policy risk. See Lead-gen agencies and The info economy.