InboxKit Pricing 2026: What Each Tier Really Costs
InboxKit runs three published tiers — Professional, Agency, Enterprise — with the per-mailbox rate falling as you climb, plus a $3/mailbox Email Warmup add-on. Here's how the bill actually comes together.
Bottom line
InboxKit's published pricing runs on annual billing across three tiers: Professional at $31/mo for 10 mailbox slots ($3.10 for each additional), Agency at $81/mo for 30 slots ($2.70 for each additional), and Enterprise at $250/mo for 100 slots ($2.50 for each additional). The effective per-mailbox cost falls from $3.10 on Professional down to $2.50 on Enterprise, so the $2.50 figure often quoted in shorthand describes the Enterprise tier specifically, not a blanket price. Base tiers include automated DNS, an admin panel with 2FA and app passwords, 24-plus sequencer integrations, placement testing, an email verifier, a blacklist checker, live monitoring, unlimited seats, full API access, and US IPs.
Email Warmup sits outside all of that as a $3/mailbox monthly add-on across every tier; Azure tenants cost $30 per domain; and the InfraGuard protection suite is a paid add-on with a free first month. Fleets above 500 mailboxes can negotiate volume pricing. The structural upside is that every tier supports Google Workspace, Microsoft 365, and Azure mailboxes from one provider, so mixing mailbox types doesn't cost you a switching premium.
How Sendbox compares
Sendbox plans start at $99/mo with dedicated IPs and the full deliverability suite included on every tier — no add-ons required.
InboxKit Plans
| Plan | Price | What you get | Watch out for |
|---|---|---|---|
| Professional billed annually, 10 mailbox slots included | $31/mo |
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| Agency billed annually, 30 mailbox slots included (Most Popular) | $81/mo |
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| Enterprise billed annually, 100 mailbox slots included | $250/mo |
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What InboxKit actually costs at common fleet sizes (annual billing)
| Usage scenario | Monthly cost | Notes |
|---|---|---|
| Solo operator, 5 mailboxes, no warmup | $31/mo | Professional's 10 slots give you room to grow, but at only 5 mailboxes the effective per-mailbox cost is $6.20 since you're under the included count. The base fee dominates the bill at this scale. |
| Solo operator, 10 mailboxes, no warmup | $31/mo | This hits the Professional slot ceiling exactly, dropping the effective rate to $3.10/mailbox — the sweet spot for this tier. |
| Solo operator, 10 mailboxes with warmup | $61/mo | $31 Professional base plus $30 in warmup (10 x $3). This is the realistic bill once warmup enters the picture, which most serious operators end up adding. |
| Small team, 30 mailboxes with warmup | $171/mo | $81 Agency base plus $90 in warmup — the Agency tier's sweet spot, landing at $5.70/mailbox all-in. |
| Growth team, 50 mailboxes with warmup | $285/mo | $81 Agency base, plus 20 extras at $2.70 each, plus warmup on all 50 at $3 each — $81 + $54 + $150. Works out to $5.70/mailbox all-in. |
| Enterprise team, 100 mailboxes with warmup | $550/mo | $250 Enterprise base plus $300 in warmup (100 x $3), landing at $5.50/mailbox all-in — noticeably better economics than the lower tiers. |
| 300 mailboxes with warmup | $1,650/mo | $250 base, plus 200 extras at $2.50 each, plus warmup across all 300 at $3 each — $250 + $500 + $900. Still $5.50/mailbox all-in. |
| 500+ mailboxes | Custom pricing | This is where the volume-discount conversation starts. Don't assume the straight sticker math holds at this size — expect to negotiate. |
How the three-tier structure actually shapes the bill
InboxKit's pricing has three breakpoints worth understanding for cost modeling. Professional at $31/mo covers up to 10 mailbox slots, with extras at $3.10 each. Agency at $81/mo covers up to 30 slots, with extras at $2.70.
Enterprise at $250/mo covers up to 100 slots, with extras at $2.50. What that means practically: each tier has a sweet spot where you land exactly on the included slot count. Hit that spot on Professional (10 mailboxes) and your effective rate is $3.10.
Hit it on Agency (30 mailboxes) and it drops to $2.70. Hit it on Enterprise (100 mailboxes) and you're at $2.50. Fall short of the sweet spot and you're wasting slots you already paid for.
Go over it and you're paying the per-extra rate, which doesn't push the overall bill down any further. Size your tier against the mailbox count you actually plan to run, not some future number. The only real path to a lower per-mailbox cost is climbing to the next tier, not stretching the one you're on.
Key takeaways
- Professional ($31): 10 mailboxes included, $3.10 per extra
- Agency ($81): 30 mailboxes included, $2.70 per extra
- Enterprise ($250): 100 mailboxes included, $2.50 per extra
- The per-mailbox rate only drops when you move up a tier, not within one
Why warmup is the $3/mailbox line most evaluators miss
InboxKit's pricing page lists Email Warmup as its own add-on, $3 per mailbox monthly, applied uniformly across all three tiers. It's probably the single most-misread line on the page, since warmup is generally treated as table-stakes for cold email rather than something optional. Run the math and it shows: a 30-mailbox Agency fleet without warmup is $81/mo; the same fleet with it is $171/mo.
A 100-mailbox Enterprise fleet without warmup is $250/mo; with it, $550/mo. That add-on line frequently equals or exceeds the base subscription itself. The practical takeaway is to budget warmup into your baseline rather than treat it as a future upgrade decision.
Two scenarios where skipping it makes sense: first, if your sending platform (Sendbox, Instantly, Smartlead) already handles warmup natively, you can route through that instead and save the add-on cost. Second, if you're only running inbound or transactional traffic, warmup may not matter much at all. For genuine cold email workflows, though, plan on paying for it.
Key takeaways
- Email Warmup is a $3/mailbox/mo add-on, not part of any base tier
- On a 30-mailbox fleet, warmup tacks on $90/mo over the $81 Agency base
- On a 100-mailbox fleet, warmup adds $300/mo over the $250 Enterprise base
- Native warmup from your sending platform can stand in for the InboxKit add-on
Provider-mixing economics across the three tiers
One genuine differentiator on InboxKit is that every tier supports Google Workspace, Microsoft 365, and Azure mailboxes under the same account, and the mailbox type you pick doesn't change the tier rate. A 30-mailbox Agency fleet split 50/50 between GWS and M365 costs the exact same $81/mo as a fleet running 100 percent GWS. That matters because cold email deliverability strategy has shifted toward mixing providers over the past couple of years.
Running 60 percent GWS alongside 40 percent M365 spreads reputation risk across two separate ESP infrastructures. Specialized providers — Zapmail's GWS-only, Mailforge's SMTP-only — force mixed-provider fleets onto multiple vendor accounts, each with its own billing, DNS flow, and panel login. On InboxKit, a mixed fleet lives on one bill and one panel instead.
The one exception is Azure, which carries a $30-per-domain fee on top of the tier economics; GWS and M365 don't trigger that charge. Teams running Azure specifically should factor that per-domain cost into their budget.
Key takeaways
- GWS, M365, and Azure all cost the same within a given tier
- Mixed-provider fleets get one bill and one panel to manage
- Azure tenants add $30/domain on top of the tier rate
- Mixing providers avoids concentrating reputation risk in one ESP
Which buyer profile fits which tier
Professional ($31/mo) suits solo operators and very small teams running under 12 mailboxes. The included 10 slots are the sweet spot; push past 12 and the per-extra rate makes Agency the cheaper all-in option. Agency ($81/mo) suits small agencies and SDR teams in the 15-40 mailbox range.
Its 30 included slots comfortably cover a typical reputation-diversified fleet for a 3-6 person outbound team, which is where most growth-stage outbound functions end up landing. Enterprise ($250/mo) suits established agencies, enterprise outbound teams, and high-volume cold email operations running 75-150 mailboxes. Its 100 included slots handle most enterprise-scale fleets without needing many extras.
Past 500 mailboxes, volume discounts get negotiated separately. Teams approaching that size should plan on a sales conversation rather than assume the published $2.50 extra-mailbox rate will hold.
Key takeaways
- Professional: solo operators running under 12 mailboxes
- Agency: small agencies and 3-6 person SDR teams, 15-40 mailboxes
- Enterprise: established agencies and enterprise outbound teams, 75-150 mailboxes
- Past 500 mailboxes, expect a custom-pricing conversation
Keep reading
Got questions? We've got answers.
Still stuck? Talk to us →Only if you're on the Enterprise tier. The published pricing runs three tiers: Professional at $31/mo (10 mailbox slots, $3.10 per extra), Agency at $81/mo (30 slots, $2.70 per extra), and Enterprise at $250/mo (100 slots, $2.50 per extra). That "$2.50" figure people quote refers specifically to the Enterprise extras rate — Professional buyers actually pay $3.10/mailbox, and Agency buyers pay $2.70.
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