Inbox supply is now a commodity sold at $2.50–3.50 per Google or Microsoft mailbox per month by a cluster of specialists (InboxKit $2.50 annual; Zapmail from $2.50; Mailforge $3 billed yearly — all as of October 2026), and below $0.50 for private-SMTP inboxes (Maildoso, $0.49 each at 1,000). Two of these vendors now sell APIs for platforms. A new sequencer does not need to own inbox infrastructure to offer "done-for-you" inboxes, and owning it carries the worst risk in the stack: getting your Google or Microsoft reseller relationship, or your IP ranges, burned by your own customers.
The strategic question is not "build or buy" but where in the chain you want to hold reputation risk. Every step towards owned infrastructure raises margin and control and concentrates the blast radius of one abusive customer.
The five models
| Model | Who does it | Your cost | Your risk | Deliverability posture |
|---|---|---|---|---|
| 1. Bring your own inboxes | Every sequencer | $0 | Low | Customer's problem |
| 2. Partner API resale | Zapmail, InboxKit APIs | $2.50–3.50/inbox/mo | Medium (vendor dependence) | Google/Microsoft inboxes, vendor-run setup |
| 3. Own reseller tenants | Google Workspace reseller, Microsoft CSP / Azure tenants | List price minus reseller margin | High (ToS, account suspensions) | Same as 2, you run it |
| 4. Private SMTP, own IPs | Maildoso, Mailforge (shared pool) | Servers + IPs + ops | High (blocklists, abuse desk) | Cheaper, not Google/Microsoft-hosted |
| 5. Dedicated IP pools as the product | EmailBison | Servers + IPs + managed ops | High, but per-customer isolated | Isolated reputation per customer |
1. Bring your own inboxes
The default. Customers buy inboxes from Zapmail, Mailforge, Maildoso, Scaledmail or directly from Google and Microsoft, then connect them. InboxKit advertises "24+ integrations" including Instantly, Smartlead, Apollo, Reply, lemlist and PlusVibe, plus "universal compatibility via app passwords" (InboxKit). The supply side already integrates with you for free; your only job is a frictionless bulk-connect flow (CSV of app passwords, or OAuth for whole tenants).
2. Partner APIs
Zapmail markets "Zapmail API: Headless Email Infrastructure for SaaS & AI SDRs" and claims 1M+ mailboxes set up, 330k+ domains and 50K+ businesses (vendor claims, Zapmail, October 2026). InboxKit offers a "full REST API with webhook notifications available across all plans" (InboxKit). Mailforge's pricing page mentions no API (Mailforge).
This lets you sell inboxes inside your checkout — domain purchase, DNS, mailbox creation, connection, warmup start — without holding a reseller agreement. Margin is whatever you mark up over $2.50–3.50; the user experience is yours.
No partner publishes wholesale or white-label API pricing, and Instantly's own done-for-you inbox price could not be retrieved for this research (its help article returned 404). The resale margin available on model 2 is unknown until you negotiate.
3. Own Google and Microsoft reseller tenants
What Zapmail, InboxKit and Primeforge do internally. You hold the Google Workspace reseller or Microsoft partner relationship and provision tenants for customers. InboxKit's price list hints at the Microsoft economics: an Azure tenant at $30 a month supporting up to 100 mailboxes (InboxKit).
The supply is fragile. Maildoso's help centre says "New or additional Google Workspace allotment is not currently available to purchase" (Maildoso help, as of October 2026). Microsoft is also moving the ground: Basic SMTP AUTH goes default-off for existing Exchange Online tenants at the end of December 2026 (MC786329), and Exchange Online caps trial tenants at 5,000 external recipients a day and onmicrosoft.com domains at 100 (Microsoft). See Provider crackdowns and Provider rules: Google, Microsoft and the ESPs.
Reseller margins for Google Workspace and Microsoft CSP, and what exactly InboxKit's "$30 Azure tenant" licenses (licensed users or unlicensed shared mailboxes), could not be verified. Whether Google is deliberately restricting Workspace supply to cold-email resellers rests on one vendor's help-centre line.
4. Private SMTP on your own IPs
Run your own MTA — Postfix (free), KumoMTA (Apache-2.0, open source; paid support from $5,000 per billion messages a year), or PowerMTA (commercial, quote-only licensing, hosting and IPs extra) — on IP ranges you lease, with IMAP from Dovecot. This is how Maildoso sells inboxes at $0.49 each at 1,000 with a 15-a-day sending limit, and how Mailforge runs a "shared IP pool" (Maildoso, Mailforge).
What you take on: IP leasing and warm-up, blocklist monitoring and delisting, PTR/HELO hygiene, feedback loops, complaint handling, and an abuse desk that can suspend a customer within the hour. Mailbox providers see private-SMTP traffic for what it is; placement at Gmail and Outlook usually trails Google- and Microsoft-hosted inboxes. See Google vs Microsoft vs SMTP and ESP matching.
5. Dedicated IP pools as the product
EmailBison sells this as its core: one plan at $599 a month for 500,000 emails, "isolated infrastructure with dedicated IPs", "single tenant clusters", "dedicated VPCs, static egress, private networking, noisy-neighbor isolation", "fully managed for you by our engineers" (EmailBison pricing, EmailBison, October 2026). Isolation answers the noisy-neighbour fear that drives agencies off shared platforms; it costs an operations team.
Cost of 1,000 sends a day of capacity
My arithmetic from list prices (October 2026), excluding domains:
| Supply | Unit price | Sends/inbox/day | Inboxes for 1,000/day | Monthly cost |
|---|---|---|---|---|
| Google/Microsoft via InboxKit or Zapmail | $2.50–3.50 | 30 | 34 | $85–119 |
| Google Workspace (Maildoso's comparison figure) | $5 | 30 | 34 | ~$165 (vendor's own figure) |
| Maildoso private SMTP | $0.49 (at 1,000) | 15 | 67 | ~$33 |
| EmailBison dedicated | $599 per 500k/mo | — | — | ~$36 |
Private infrastructure is three times cheaper per send than Google- or Microsoft-hosted inboxes. The question is whether it lands. That is a deliverability question with contested evidence (see Google vs Microsoft vs SMTP, Reputation and blocklists, Contradictions register).
Owned infrastructure is a margin play that only pays once you have volume and a mature abuse operation. Before that, it is a liability that sits on your balance sheet and your IP ranges. The players that own infrastructure (EmailBison, Mailforge alongside Salesforge, Maildoso) run it as their core business with dedicated operations, not as a side feature of a sequencer.
A single wave of abusive customers can get leased IP ranges listed, or a Workspace reseller account suspended, taking down every honest customer on the same infrastructure. Without know-your-customer checks, sending caps for new accounts and fast suspension, owned infrastructure is the fastest way to end the company.
Recommended sequence for a new entrant
- Months 0–6: bring-your-own inboxes only, with best-in-class bulk connect (CSV app passwords, tenant-wide Microsoft OAuth).
- Months 6–12: embed one or two partner APIs (InboxKit, Zapmail) for in-app inbox purchase; mark up modestly; own the onboarding UX.
- Year 2+: if 30%+ of volume buys inboxes through you, consider your own Microsoft tenants or a dedicated-IP tier for agencies, built on KumoMTA, priced like EmailBison.
What this means for an entrant
- Do not build inbox infrastructure first. Supply is commoditised at $2.50–3.50 per inbox (October 2026) and already integrates with every sequencer; your differentiation is not there.
- Partner APIs give you done-for-you inboxes without reseller risk. Negotiate wholesale with InboxKit or Zapmail and make inbox purchase part of onboarding — it lifts revenue per account and cuts time-to-first-send.
- If you go private-SMTP, use KumoMTA over PowerMTA. It is open source (Apache-2.0) with published support pricing; PowerMTA is quote-only.
- Dedicated, isolated infrastructure is the one infra product with a clear buyer: agencies burned by shared-pool neighbours. EmailBison proves the price point ($599/mo for 500k emails); an EU-hosted variant is an opening for a German founder.
- Watch Microsoft's December 2026 SMTP AUTH change and Google Workspace supply — they reprice every model above. See OAuth verification and the end of basic auth and Inbox & domain infrastructure.