How much does it cost to give 10,000 users their own inbox?
Published · 8 min read
TL;DR
At list prices, 10,000 agent inboxes cost $4,309 a month on Nylas, $10,417 on Resend and $19,900 on AgentMail Startup, because each bills per inbox. OpenEmail (open.email) bills mail, not mailboxes: about $191 a month, after a limit raise.
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Prices checked 10 November 2026.
If your product gives an email address on your own domain to every agent, person or task (one per customer, per support case, per deal), you end up with a large number of low-volume inboxes. How a service prices inboxes then decides most of the bill. Below is the math for 1,000, 10,000 and 100,000 inboxes.
What do 10,000 inboxes cost on each service?
At 10,000 agent inboxes, AgentMail, Resend and Nylas cost $4,309 to $19,900 a month, and OpenEmail about $191. The inbox fee is 92 to 99% of each of their bills.
To compare like with like, every service gets the same usage, the profile we use when we quote agent customers. Each inbox receives 20 emails and sends 40 a month. It sends more than it receives because an agent writes to people first and not every message gets a reply. An incoming email is 35 KB on average. An outgoing one counts as 52.5 KB, because we assume 1.5 recipients per message for the times someone else is copied. Mail is kept 30 days. Prices are list prices with monthly billing, in US dollars, without discounts or tax. The totals are our arithmetic on public prices, not vendor quotes.
| Inboxes | OpenEmail | Nylas Agent Accounts | Resend Inboxes | AgentMail |
|---|---|---|---|---|
| 1,000 | $32 | $429 | $1,026 | $1,900 |
| 10,000 | $191 | $4,309 | $10,417 | $19,900 |
| 100,000 | $1,936 | $43,465 | $102,757 | $199,900 |
| Per inbox, at 10,000 | $0.02 | $0.43 | $1.04 | $1.99 |
| Fee per inbox | none | $0.40 after 50 | $1 after 3 | $2 after 150 |
Notes on the table
Every OpenEmail figure here is above Pro's default send limit of 20,000 a month, so the account's limits have to be raised first (see the last section). AgentMail figures use the Startup list price. At this size AgentMail also offers Enterprise, with "Custom" pricing and "Unlimited inboxes available", and at 100,000 inboxes the sends pass Startup's listed 15,000 emails a day.
The totals come from public list prices. Nylas Agent Accounts Pro is $49 a month with 50 agent accounts, then $0.40 per account. It includes 50,000 emails, then $0.80 per 1,000, and 50 GB each of storage and bandwidth, then $0.40 and $0.80 per GB. Nylas counts only sent emails, while bandwidth covers mail in both directions. Volume band pricing is listed only for Enterprise, so at 100,000 inboxes a negotiated Nylas price could be lower than our Pro figure.
Resend gives 3 inboxes on every plan. With pay-as-you-go on a paid plan, each extra inbox is $1 a month, and inbox emails, sent and received, use the plan's email quota. The rate per 1,000 emails falls on bigger plans, from $0.90 on Pro to $0.46 on the 2,500,000-email plan, and the totals use the cheapest plan for each case. AgentMail Startup is $200 a month with 150 inboxes, then $2 per extra inbox a month.
OpenEmail has no fee per mailbox. Pro is $20 a month, and after that you pay only for sends, receives, forwards and storage.
Why does the bill grow with every inbox?
AgentMail, Resend and Nylas charge per inbox, so each new inbox adds a fee regardless of how much mail it gets.
The fee comes on top of the plan: $2 per inbox past 150 on AgentMail Startup, $1 past 3 on Resend, $0.40 past 50 on Nylas Pro. At a few dozen inboxes this is small. At 10,000 it is most of the bill, because the mail itself is cheap next to 10,000 fees.
OpenEmail is priced on messages and storage, never per mailbox. You pay for mail, not for each address, so the same traffic spread over more mailboxes costs the same. At 10,000 agent inboxes that works out to about 2 cents per inbox a month.
What changes at 1,000 and at 100,000 inboxes?
At 1,000 inboxes, AgentMail, Resend and Nylas cost $429 to $1,900 a month and OpenEmail $32. At 100,000 inboxes they cost $43,465 to $199,900, and OpenEmail about $1,936.
The gap grows with the inbox count, and each service hits a different limit first. On Nylas, $0.40 per account past 50 comes to $39,980 a month at 100,000 inboxes, before any email.
AgentMail Startup lists 15,000 emails a day. 10,000 agent inboxes send about 13,300 a day, just under it, or about 20,000 if received mail also counts (the page does not say). 100,000 inboxes send about 133,300 a day. The $2 sends add-on (1,000 emails a month) raises the daily limit.
On OpenEmail the price does not depend on the mailbox count, but the default account limits do not cover this volume. They have to be raised first, as the last section explains.
How is OpenEmail's price calculated?
OpenEmail Pro is $20 a month with 10,000 sends, 50,000 receives and 30 GB of storage included. Above that: $0.40 per 1,000 sends, $0.10 per 1,000 receives, $0.30 per 1,000 forwards, $0.50 per GB a month.
Three details decide what you actually pay. A message counts as one receive at each of your addresses it reaches. A webhook copy of a message does not add a receive, so you can get the message as JSON and still keep it in the mailbox. And storage is billed on the daily average, so a short spike barely moves the bill.
Here is the full calculation for 100,000 agent inboxes, each receiving 20 emails and sending 40 a month. Note that the number of mailboxes is not part of it.
| Item | Usage | Included in Pro | Billed | Rate | Cost |
|---|---|---|---|---|---|
| Base (Pro) | $20.00 | ||||
| Sends | 4,000,000 | 10,000 | 3,990,000 | $0.40 per 1,000 | $1,596.00 |
| Receives | 2,000,000 | 50,000 | 1,950,000 | $0.10 per 1,000 | $195.00 |
| Storage | 280 GB | 30 GB | 250 GB | $0.50 per GB | $125.00 |
| Total per month | $1,936.00 |
Sends are most of it, $1,596 of $1,936. They are OpenEmail's most expensive meter, $0.40 per 1,000 against $0.10 for receives, so an agent that mostly writes will pay mostly for sends. Storage is the other number to watch: it is $125 here with mail kept 30 days, and it grows if you keep mail longer.
All rates and limits are on the pricing page and in Account limits.
What if the inboxes mostly receive?
With 20 received and 5 sent per inbox a month, 10,000 inboxes cost about $51 on OpenEmail and $4,029 to $19,900 on AgentMail, Resend and Nylas.
This is the shape of a mailbox per customer that mostly collects confirmations, receipts and support mail. With fewer sends, the OpenEmail column is about four times smaller from 10,000 inboxes up. The per-inbox bills barely change, because at 10,000 inboxes the fee is 98 to 99% of each of them.
| Inboxes | OpenEmail | Nylas Agent Accounts | Resend Inboxes | AgentMail |
|---|---|---|---|---|
| 1,000 | $20 | $429 | $1,017 | $1,900 |
| 10,000 | $51 | $4,029 | $10,167 | $19,900 |
| 100,000 | $440 | $40,434 | $101,147 | $199,900 |
Notes on the table
All emails here are 35 KB. From 10,000 inboxes the OpenEmail figures are above Pro's default receive limit of 100,000 a month, so the account's limits have to be raised first.
When is OpenEmail not the best fit?
The comparison above favours many inboxes with modest traffic, and two cases look different.
With only a few low-volume inboxes, price does not decide. AgentMail, Resend and Nylas each include 3 inboxes or agent accounts free, and OpenEmail Free is $0. And if you only send mail and never need an inbox, a sending service fits better.
What do I need to do before creating 10,000 mailboxes?
Get the account approved, plan for 5,000 new mailboxes a day, and request higher send and receive limits before launch.
The price in the tables assumes the account can already handle the volume, but a new account starts much lower. Every account starts at 3 mailboxes until an internal approval. No approval time is published, so start early.
After approval, a Pro account creates 5,000 mailboxes a day. Past that, the API answers 429 rate_limited until the rolling 24-hour window frees up, so 100,000 mailboxes take 20 days. To create more per day, email hello@open.email before you start.
The Pro defaults are also below this volume. Receives are capped at 1,000 an hour, 10,000 a day and 100,000 a month; sends at 200 an hour, 2,000 a day and 20,000 a month. 10,000 agent inboxes already need 200,000 receives and 400,000 sends a month. OpenEmail can raise Pro limits per account, but no ceiling or turnaround is published, so email support@open.email with your numbers before launch.
This matters before launch. Receiving over a limit defers mail instead of bouncing it, and the mail arrives once the window has room. But sending servers retry on their own schedule and eventually give up, so a limit that stays hit for days can still cost you mail.
Once the limits are in place, running thousands of mailboxes is simple. Mailboxes are created and deleted by API on demand, so one can start with the work and be deleted when it ends. One account API key works for every mailbox, so you don't manage a password per mailbox to use the API. And if the addresses only need to receive, a pattern route (for example order-*) sends all matching addresses to one mailbox or webhook, without creating a mailbox per address.
FAQ
Do I need 10,000 actual email accounts?
No. One OpenEmail account holds every mailbox. A catch-all address (*) can also collect mail for every address on a domain.
Do AgentMail, Resend or Nylas give volume discounts? AgentMail lists Enterprise as custom with usage-based pricing. Resend's rate per 1,000 emails falls on larger plans. Nylas lists volume band pricing on Enterprise.
Does AgentMail publish a price for 10,000 inboxes? Not directly. Startup's $2 add-on rate gives $19,900 a month; Enterprise says "Unlimited inboxes available" at a custom price.
Do received emails cost money? On OpenEmail, $0.10 per 1,000 past the 50,000 included in Pro. On Resend, received inbox emails use the plan's email quota. Nylas counts only sent emails but bills bandwidth in both directions.
Can people open these mailboxes in a mail app? On OpenEmail, yes. Each one is a real mailbox: agents and apps use the API, people use webmail at webmail.open.email or any client over IMAP, POP3, SMTP or JMAP. Same mailbox, nothing copied.
Sources
Checked 10 November 2026 unless noted.
- AgentMail pricing: https://www.agentmail.to/pricing
- Resend pricing: https://resend.com/pricing
- Nylas pricing: https://www.nylas.com/pricing/
- Nylas Agent Account usage limits (read 2 November 2026): https://developer.nylas.com/docs/v3/agent-accounts/send-limits/
- OpenEmail pricing: https://open.email/pricing
- OpenEmail account limits: https://open.email/docs/guides/account-limits