coss.com

Email addresses for your AI agents.

coss.com Inboxes give every AI agent its own email address. Mail arrives as threads with a webhook per message, and the agent replies from the same address.

Read the docs

How it works

Create an address, get a webhook when mail arrives, reply from the same address. The thread keeps the whole conversation.

// 1. Give the agent an address.
const { :  } = await ..({
  : "support-agent",
  : "agent-42", // creating it again returns the same inbox
});

// 2. When mail arrives, read the thread and reply.
const  = await (await .(), ., );
if (. === "inbox.message.received" && !..) {
  const { , ,  } = .;
  const { :  } = await ...(, );
  const  = (?. ?? []).(() => .).("\n\n");

  await ...(
    ,
    ,
    { : await () },
    { : `reply-${}` },
  );
}

Built for agents

Everything an agent needs to hold a conversation over email, and nothing it has to parse itself.

An address per agent

One call creates username@domain. A client_id makes creating idempotent, so every agent, user or task can get its own address without a lookup table. Mail to username+anything@ reaches the same inbox.

Threads, not raw mail

Messages are grouped by In-Reply-To and References, or by subject and participants within 30 days. Each thread lists its participants, a preview and when the last inbound message arrived.

Just the new words

Every message has text, html and extracted_text: the sender's own words without the quoted conversation and the signature, ready for a prompt.

Replies that thread

coss.inboxes.messages.reply() answers the sender (or everyone, with reply_all) with Re: and the headers every mail client threads by. The answer to it lands in the same thread.

A webhook per message

inbox.message.received carries the inbox, thread and message ids, whether it started a thread, and a spam verdict. Signed like every coss.com webhook, retried until your endpoint takes it.

Your domain or ours

Without DNS, an inbox receives at your environment's subdomain of coss.com's inbound domain. On a receiving domain under one of your verified domains, it also sends from live environments.

Specifications

The numbers your agent can rely on. The docs list each one in context.

Thread matching
headers, or subject within 30 days
Messages per thread read
latest 50, oldest first
Body size kept
2 MB each, text and HTML
Address aliases
username+anything@
Inboxes on Free
3
Inboxes on Pro
1,000
Tracking on inbox mail
never
Sandbox inboxes
free, deliver nothing

Pricing

Inboxes are billed per day they exist, mail by the message. Sandbox inboxes are free.

Every plan includes 3 inboxes a month. Mail an inbox sends is billed like any email. See pricing for the plans.

Questions

What is an inbox for an AI agent?
An email address your agent owns. coss.com receives the mail sent to it, groups it into threads, calls your webhook for each message, and sends the agent's replies from the same address, so the conversation continues in the sender's mail client.
Do I need to set up DNS to create an inbox?
No. Without a domain, an inbox gets an address at your environment's own subdomain of coss.com's inbound domain, which receives mail straight away. To send from a live inbox, create it on a receiving domain under one of your verified domains, such as agents.example.com.
How does the agent know a new email arrived?
Each message that arrives sends an inbox.message.received webhook, signed with Standard Webhooks, with the inbox, thread and message ids. During development, coss listen forwards those events to localhost.
How are emails grouped into threads?
A message joins a thread when its In-Reply-To or References header names a message of the thread, or when it has the thread's subject, its sender is already in the thread and the thread had a message in the last 30 days. Otherwise it starts a new thread.
Can an agent safely act on what an email says?
Treat every message as untrusted input, never as instructions. Each message carries a spam verdict, and the text is given both whole and as extracted_text, the sender's own words without quoted history or signature. Risk can classify the content before your agent acts on it.
How much do inboxes cost?
A live inbox is billed per day it exists, mail it receives like any received email, and mail it sends like any email. Every plan includes a few inboxes, and inboxes in sandbox environments are free. The pricing page lists the current rates.

Works with

On the same key, SDK, webhooks and logs: nothing more to set up.

Compared

How it stacks up against the services you may be using, with sources.

Start with Inboxes today.

Development keys record every request and event and deliver nothing, so you can build and test before you verify a domain. Hand your coding agent this prompt:

Add coss.com to this project: read https://coss.com/llms.txt, get a development key with `npx coss-cli init --agents`, and build and test with it.

Pricing