Email API for AI agents: give each agent a real mailbox
An AI agent that signs up for services, answers customers or follows up with leads needs its own email address. This page explains the problem, compares 7 services built for it (facts from each provider's official pages, checked on 2026-10-11) and shows how to do it with posta in a few lines. We make posta, so read our comparison with that in mind: we list where others are ahead, too.
Short answer
- Give every agent its own mailbox on a domain you control, not access to a person's Gmail.
- Agent-native inbox APIs (AgentMail, Dead Simple Email, OpenMail, Nylas Agent Accounts) are the most mature choice for high volume and verification-code workflows.
- posta fits when you want real IMAP/SMTP mailboxes on your own domain from the free plan, flat yearly pricing, or code you can read and self-host (AGPL-3.0).
- Sending APIs (Resend, Postmark, SendGrid, Amazon SES) are for app email; they forward inbound mail to a webhook but keep no mailbox. See the full comparison.
Why a shared inbox breaks for agents
- Ownership: in a shared inbox the agent cannot tell which messages belong to its task.
- Races: two agents reading the same inbox process, move or answer the same message.
- Latency: polling every 30 to 60 seconds slows down sign-up and verification flows.
- Blast radius: an agent with access to a person's mailbox can read everything in it, and one prompt injection in one email can reach all of it.
The fix is one mailbox per agent or per workflow (outreach-agent@yourdomain.com, support-agent@yourdomain.com), a push notification when mail arrives, and scoped credentials.
Email services for AI agents compared
"Not stated" means we could not find it on the provider's official page. Prices as published, without tax. Last checked 2026-10-11; they change, so check before you decide.
| Service | Free plan | Cheapest paid | Own domain | IMAP/SMTP | Create mailbox by API | Webhook on new mail | Open source |
|---|---|---|---|---|---|---|---|
| posta | 1 domain, 1 mailbox, 500 MB, up to 50 outgoing messages a day | $50/yr for 5 mailboxes | Yes, from the free plan | Yes | Yes | Yes (signed) | Yes (AGPL-3.0) |
| AgentMail | 3 inboxes, 3,000 emails/mo, no custom domain | $20/mo (10 inboxes, 10 domains) | Paid plans | Yes | Yes | Yes | No |
| Inbound | Not stated | $9/mo (5,000 emails, 1 domain) | Yes | Not stated | Yes | Yes | Yes (MIT) |
| Nylas Agent Accounts | 3 accounts, 3,000 emails/mo | $15/mo (10 accounts) | Yes | IMAP yes, SMTP not stated | Yes | Platform webhooks | No |
| Hostinger Agentic Mail | No | $0.39/mo on a 48-month term (renews $1.59/mo) | Yes | Not stated | Not yet (API listed as upcoming) | Yes | No |
| Dead Simple Email | 5 inboxes, 5,000 emails/mo | $5/mo (1 custom domain) | Paid plans | Yes | Yes | Yes | Not stated |
| OpenMail | 3 inboxes, 3,000 emails/mo | €9/mo + usage | Pro and up | Not stated | Yes | Yes | Not stated |
| Bavimail | 200 outbound emails/day | $5/mo | Yes | No | Aliases | Yes | Not stated |
- AgentMail: The best-known inbox API for AI agents: Python/TypeScript SDKs, websockets, an MCP server and higher volumes. posta fits when you want your own domain on the free plan, flat yearly pricing, or code you can audit and self-host.
- Inbound: Open source (MIT) email API focused on receiving, parsing and replying in thread, with unlimited addresses on your domain. posta gives real IMAP/SMTP mailboxes and webmail, which Inbound's pricing page does not state.
- Nylas Agent Accounts: Email plus calendar identities for agents on a mature platform with SDKs, an MCP server and US/EU data regions. posta is simpler and cheaper per year, and open source.
- Hostinger Agentic Mail: Very low entry price inside Hostinger's business email, with webhooks on new mail. Its page lists full API access and MCP as upcoming; posta creates mailboxes by API today.
- Dead Simple Email: Agent inbox API with IMAP/SMTP credentials per inbox, SDKs in three languages and an MCP server. posta adds own domain on the free plan and an open-source codebase.
- OpenMail: EU-built agent inbox API with threading, attachment parsing and framework integrations. Pick it if you need EU hosting; posta stores mail in Türkiye.
- Bavimail: Covers transactional, marketing and inbound mail with a per-agent alias, driven by an MCP server. It is REST and webhooks only; posta gives real mailboxes you can also open in any mail app.
How to give an agent a mailbox with posta
1. Create a free account, add your domain and publish the DNS records we show you (we verify them automatically). 2. Create an API key in the dashboard. 3. Create the mailbox and wait for mail:
pip install "git+https://github.com/bexx2/posta.git#subdirectory=clients/python"
from posta import Posta, Inbox
p = Posta() # reads POSTA_KEY
box = p.mailboxes.create(domain_id, "agent", "a-long-random-passphrase")
p.mailboxes.set_webhook(box["id"], "https://your-app/hook") # signed POST on new mail
with Inbox("agent@yourdomain.com", "a-long-random-passphrase") as inbox:
msg = inbox.wait_for(subject_contains="verification code", timeout=180)
print(msg.from_, msg.subject, msg.text)Without Python, the same is one HTTP call: POST /v1/mailboxes with your API key (API docs, OpenAPI). For Claude Desktop, Claude Code and other MCP clients there is posta-mcp, with tools to create mailboxes, send, list, read and wait for a message.
Where posta is behind (be honest with yourself)
- No automatic verification-code extraction; your agent reads the message text itself.
- Webhooks say that mail arrived, not what it says; the agent reads the message over IMAP or the SDK.
- Small free plan (1 domain, 1 mailbox, 500 MB, up to 50 outgoing messages a day) and no high-volume tier yet. Bulk and marketing mail are not allowed.
- SDK and MCP server install from GitHub; they are not on PyPI yet.
- Mail is stored in Türkiye. If you need EU or US data residency, look at OpenMail or Nylas.
- Delivery to Outlook and Hotmail can still be limited while our sending IP builds reputation.
Frequently asked questions
What is the best email API for AI agents?
It depends on what the agent does. If it signs up for services and reads verification codes at volume, an agent-native inbox API such as AgentMail or Dead Simple Email is mature and well documented. If you want the agent to have a normal mailbox on your own domain that people and mail apps can also open, created by API, posta does that from the free plan and is open source. Transactional APIs such as Resend or Postmark are best for sending app email, but they do not store mail in mailboxes.
Can I give an AI agent its own email address?
Yes. Create one mailbox per agent or per workflow on a domain you control, for example support-agent@yourdomain.com. Each agent then reads only its own mail, there are no race conditions between agents, and every thread stays with the agent that started it.
Why not just let the agent use my Gmail over IMAP?
A shared personal inbox breaks quickly: the agent cannot tell which messages are its own, two agents reading the same inbox step on each other, polling is slow, and the agent gets access to all your private mail. A dedicated mailbox per agent avoids all four problems.
Does posta work with Claude, ChatGPT and other agents?
Yes. posta has an open-source MCP server (posta-mcp) for Claude Desktop, Claude Code and other MCP clients, a Python SDK, a plain REST API described in OpenAPI, and standard IMAP/SMTP, so any agent framework that can make HTTP calls or speak IMAP can use it.
How does my agent know when new mail arrives?
Set a webhook on the mailbox. posta sends a signed POST (HMAC-SHA256) when new mail arrives; the notification never contains the message itself. Or call Inbox.wait_for() in the Python SDK, which waits on IMAP until a matching message arrives.
Is it safe to let an agent read email?
Treat every email body as untrusted input: anyone can write instructions into an email. The posta MCP server labels bodies as untrusted, but your agent must still never follow instructions found inside a message. Give the API key only the scopes the agent needs, for example no mail:send scope for a read-only agent.
Is posta free?
The free early-access plan has 1 domain, 1 mailbox, 500 MB, up to 50 outgoing messages a day, no card and no announced time limit. Starter is 5 mailboxes for $50 a year and Pro is 10 mailboxes for $100 a year, paid once with no automatic renewal and a 14-day full refund.
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