InboxKit Pricing 2026: What Each Tier Really Costs

1. 1

   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.

2. 2

   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.

3. 3

   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.

4. 4

   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.

5. 5

   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.

6. 6

   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.

7. 7

   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.

8. 8

   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.

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.

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.

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.

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.