Resend Inboxes gives Resend customers a hosted, threaded inbox at an address on their own verified domain, with access through the API, SDKs, CLI and MCP, plus an optional built-in agent. It fits teams that already send with Resend, own the domain, and want people and agents working the same inboxes. An inbox-per-agent API such as SentFromAI fits better when your product creates many addresses, needs them to work without DNS changes, or when inbox count drives cost. At low email volume, 100 inboxes cost $117 a month on Resend Pro and $19 a month on SentFromAI Pro.
This comparison uses Resend's announcement, documentation and pricing as checked on October 27, 2026. Prices and limits change, so confirm them before you commit.
What Resend launched
Resend started as a sending API and added receiving in 2025. Receiving fired a webhook for each inbound message, and in Resend's words, "the rest is up to you." In late October 2026, Resend announced Inboxes to cover that work, positioned as "built for agents, but ready for your whole team."
According to the announcement and Inboxes documentation, an inbox includes:
- an address on one of your verified domains that sends and receives, with every message grouped into a thread;
- built-in
inbox,sent,archive,trashandspamfolders; - search with a Gmail-like query syntax, plus sender and date filters;
- labels, drafts, forwarding, and reply all with CC, BCC and idempotency keys;
- webhooks for inbox events, from
inbox.email.receivedtoinbox.thread.assigned; - team features: shared inboxes, thread assignment and notes;
- access through the API, official SDKs, the CLI and Resend's MCP server; and
- a built-in agent you configure with instructions, a tone and a list of allowed actions, such as drafting replies, adding labels, forwarding, assigning, marking spam, archiving and deleting.
Resend's pricing includes 3 inboxes on every plan. Paid plans can enable pay-as-you-go and add inboxes at $1 per inbox per month; the Free plan stops at 3. Inboxes have no separate email allowance. Every email sent or received through an inbox counts toward the plan's transactional quota, which is 3,000 emails a month on Free (with a 100-sends-a-day limit) and 50,000 on the $20 Pro tier.
What changes for AI-agent developers
"Resend only sends email" is now out of date. If you ruled Resend out because it did not store or thread conversations, re-evaluate it.
The decision now turns on the model each product is built around rather than a missing feature:
- Address ownership. Resend inboxes live on your domain. An inbox-per-agent API can also issue addresses on a shared provider domain.
- Who works the inbox. Resend designs for people and agents in the same inbox. Agent inbox APIs design for software that creates and uses inboxes programmatically.
- Pricing unit. Resend prices by email volume and adds inboxes at $1 each. SentFromAI prices plans by inbox count with an email allowance.
Resend Inboxes vs an inbox-per-agent API
| Decision area | Resend Inboxes | SentFromAI |
|---|---|---|
| Address | On your verified domain. Receiving needs an MX record, or forwarding from an existing mailbox | On the shared mail.sentfrom.ai domain, live immediately with no DNS setup. Custom domains on Pro and up |
| Inboxes included | 3 on every plan. $1 per extra inbox per month on paid plans | 5 on Free, 100 on Pro, 1,500 on Scale, 15,000 on Business |
| Email allowance | Inbox email, sent and received, uses the transactional quota: 3,000 on Free, 50,000 on the $20 Pro tier | 5,000 on Free, 20,000 on Pro, 200,000 on Scale, 600,000 on Business |
| Conversations | Threads, folders, labels and drafts | Threads rebuilt from Message-ID, In-Reply-To and References, plus labels and drafts |
| Search | Gmail-like query syntax with sender and date filters | Keyword on all plans. Semantic and hybrid on Pro and up |
| Inbound events | Inbox webhooks | Signed webhooks, a realtime WebSocket stream, or polling |
| People in the loop | Shared inbox UI, assignment and notes | API-first. Thread assignment and notes are not documented, so review flows live in your app |
| Built-in agent | Yes, configured by instructions and allowed actions | No. Bring your own agent through the REST API, MCP server or framework guides |
| History | Pricing lists 30-day data retention on Free, Pro and Scale. Confirm how it applies to inbox threads | Storage allowance per plan, from 5 GB on Free to 600 GB on Business |
| Hosting location | Multi-region sending on all plans. Confirm where inbox data is stored | Email processing, storage, attachments and backups in Ireland on every plan |
Both products send and receive, keep threads, support drafts and labels, and connect to agents through MCP. Neither table column is a universal winner.
What it costs at different fleet sizes
The figures below use list prices as of October 27, 2026, before tax. They take the cheapest plan that covers both inbox count and monthly email volume, with Resend pay-as-you-go enabled. Volume counts mail sent and received, and each scenario stays inside both plans' allowances. Resend's API documents a per-plan inbox limit but its pricing page does not publish one for pay-as-you-go plans, so confirm limits for large fleets with each provider.
| Scenario | Resend | SentFromAI |
|---|---|---|
| 3 inboxes, up to 3,000 emails a month | $0 (Free, up to 100 sends a day) | $0 (Free) |
| 25 inboxes, up to 20,000 emails a month | $42 (Pro $20 + 22 inboxes) | $19 (Pro) |
| 100 inboxes, up to 20,000 emails a month | $117 (Pro $20 + 97 inboxes) | $19 (Pro) |
| 1,000 inboxes, up to 100,000 emails a month | $1,032 (Pro $35 tier + 997 inboxes) | $99 (Scale) |
| 5 inboxes, 100,000 emails a month | $37 (Pro $35 tier + 2 inboxes) | $99 (Scale) |
The pattern is consistent. Resend costs less when a few inboxes carry a lot of mail. SentFromAI costs less as the number of inboxes grows, because its plans include inboxes in bulk.
Choose Resend Inboxes when
- You already send with Resend and want inboxes from the same vendor, key and dashboard.
- Every address should be on your own domain, and you can add an MX record or set up forwarding.
- People and agents share the same inboxes and need assignment, notes and a UI.
- A small number of inboxes carry high email volume.
- You want a hosted triage agent configured by instructions instead of running your own agent loop.
- Your stack needs an official SDK in Node.js, PHP, Python, Ruby, Go, Rust, Java or .NET.
Choose an inbox-per-agent API when
- Your application creates an address per agent, customer, task or tenant, and the count can reach hundreds or thousands.
- A new address must work as soon as it is created, without a DNS change. SentFromAI addresses on
mail.sentfrom.aiare live immediately, and custom domains stay optional. - Inbox count, not send volume, drives your costs.
- You run your own agent and want email as tools. SentFromAI's MCP server exposes 21 tools, and its docs include guides for the Anthropic Messages API, OpenAI Agents SDK, LangChain, CrewAI and others.
- You need EU-hosted email on every plan, including Free.
Test the workflow before you commit
Run the same checks against either provider:
- Provisioning. Create inboxes through the API at your expected rate, and confirm the error at the limit. Resend returns
403at a plan's inbox limit; SentFromAI returns402 inbox_limit_reached. - A full conversation. Send a message, receive a real reply, and confirm both sides see one thread. Our threading guide covers the headers involved.
- Retries. Retry a send after a timeout. Resend lists idempotency keys for inbox replies and forwards; SentFromAI returns the original message for a repeated
client_id. See retry-safe sending. - Events. Verify webhook signatures, replay a delivery, and confirm your handler processes it once. Compare push and pull options in webhooks vs WebSockets vs polling.
- Untrusted input. Treat inbound email as data, not instructions, and keep sending behind a policy you control. See prompt injection by email.
- Retention and location. Confirm how long threads are kept and where they are stored before production data arrives.
Where SentFromAI fits
SentFromAI is email infrastructure for AI agents: inboxes you provision per agent, two-way email, threads, drafts, search and signed inbound events. The email product page describes the model, pricing lists each plan's limits, and how to give an AI agent its own email address walks through setup. For another platform comparison, see Telnyx Email API vs an agent inbox API.
You can start free with 5 inboxes and 5,000 emails a month, then run the send, receive and reply test above before deciding.
Frequently asked questions
Does Resend support inboxes for AI agents?
Yes. Resend Inboxes host threaded inboxes on your verified domains, and agents can use them through the API, CLI or MCP server. Resend also offers a built-in agent that acts on inbound mail within the actions you allow.
Do Resend Inboxes need a custom domain?
Yes. The inbox address must be on a domain verified for sending. To receive directly, the domain needs receiving enabled with an MX record. If you cannot change the MX record, create the inbox with forwarding enabled and forward an existing mailbox to the receiving address Resend provides.
How much do Resend Inboxes cost?
Every Resend plan includes 3 inboxes. Paid plans with pay-as-you-go add inboxes at $1 per inbox per month, and the Free plan stops at 3. Every email sent or received through an inbox counts toward the plan's transactional email quota.
Is Resend Inboxes the same as Resend receiving?
No. Receiving delivers inbound messages for a domain to your own webhook endpoint. Inboxes store, thread and organize conversations for you.
Which is cheaper for 100 AI agents?
At up to 20,000 emails a month, 100 inboxes cost $117 a month on Resend Pro ($20 plus 97 extra inboxes) and $19 a month on SentFromAI Pro, which includes 100 inboxes. With only a few inboxes and high volume, Resend is usually cheaper.
Sources
Checked October 27, 2026.
- Resend, Announcing Inboxes: features, built-in agent actions, MCP access and inbox pricing.
- Resend, Pricing: plan quotas, inbox pricing, quota counting, data retention and multi-region sending.
- Resend, Inboxes documentation: folders, prerequisites and the difference from receiving.
- Resend, Creating Inboxes: domain, MX record and forwarding requirements.
- Resend, Create Inbox API:
422,409and403responses. - SentFromAI, Pricing: plans, inbox counts, email allowances and storage.
- SentFromAI, Inboxes: shared domain, custom domains and
402 inbox_limit_reached. - SentFromAI, Threads and messages: header-based thread reconstruction.
- SentFromAI, Receiving email: signed webhooks, retries and polling.
- SentFromAI, Realtime WebSocket: live event stream.
- SentFromAI, Search: keyword, semantic and hybrid modes by plan.
- SentFromAI, FAQ: immediate shared-domain addresses and
client_idretries. - SentFromAI, MCP guide: MCP server and tools.
- SentFromAI, EU-hosted email for AI agents: hosting scope.