How to Connect ChatGPT to Cold Email

Short answer: ChatGPT reaches outside tools in two ways. Custom MCP connectors arrived on 4 June 2025, and developer mode later added full read and write MCP support; both need a public MCP server that signs in with OAuth or needs no auth. Outreach2day's hosted MCP server is live with OAuth sign-in (since 29 September 2026), so in developer mode you add https://public.outreach2day.com/mcp and sign in with your Outreach2day account, no API key. GPT Actions let a custom GPT call a REST API from an OpenAPI schema, with an API key; this also works with the Outreach2day API today; Codex CLI adds the MCP server with one installer command and signs in with codex mcp login; the OpenAI Responses API takes remote MCP servers with headers per OpenAI's docs, not tested by us yet. One caveat: on 11 September 2026 OpenAI announced a planned retirement of custom GPTs, with a migration path to plugins, so treat a GPT Action as the setup for now, not for years.

Checked September 2026 against the ChatGPT release notes, GPT Actions, GPT Action authentication and Developer mode and MCP apps in ChatGPT.

Connection Methods

The quickest route is developer mode: open Settings, Security and login, turn on Developer mode, then at chatgpt.com/plugins click +, create an app with the server URL https://public.outreach2day.com/mcp and sign in with your Outreach2day account. ChatGPT keeps the sign-in; there is no API key to paste. The GPT Action below is the REST API route, for plans without developer mode.

Per OpenAI's help center, developer mode with full MCP support is rolling out in beta to Business, Enterprise and Edu workspaces on the web, where admins or owners turn it on. OpenAI's developer documentation also lists Pro and Plus accounts; check what your plan shows under Settings.

What ChatGPT Can Do With Your Cold Email Setup

A GPT Action can call only the operations in the schema you give it. Start with reads:

Leave POST /domains/purchase out of the schema unless you do. New mailboxes are ordered in the app. The campaign endpoints are not enabled for every workspace, so a GPT that promises to "launch the campaign" would fail for most accounts.

GET /mailboxes also returns each mailbox's SMTP/IMAP password. A GPT Action cannot filter fields, so either leave that operation out and use warmupMailboxes for status, or put a small proxy in front that drops the field.

Connect ChatGPT to Outreach2day in developer mode, step by step

1. Turn on developer mode. Settings, Security and login, Developer mode; availability depends on your account and workspace policy

2. Create the app. At chatgpt.com/plugins click +, name it Outreach2day and enter https://public.outreach2day.com/mcp under Connection

3. Sign in. In a new chat, add Outreach2day from the tools menu; ChatGPT opens the Outreach2day sign-in page on the first tool call

4. Ask a read-only question. Ask "Which mailboxes are still warming?" and compare the answer with the app

Connect ChatGPT with a GPT Action (REST API)

1. Create an API key. In the Outreach2day app, open the API keys page and create a key only for this GPT, so you can revoke it on its own

2. Trim the OpenAPI schema. Download https://public.outreach2day.com/openapi.json and keep only the operations in the table above; a short schema gives the model fewer ways to go wrong

3. Create the GPT. In ChatGPT, open the GPT editor, go to Configure, and add an Action. Paste the trimmed schema; the server URL is https://public.outreach2day.com

4. Set authentication. Choose API Key, pick the Bearer auth type so the key goes in the Authorization header, and enter the key in OpenAI's key field; OpenAI stores it encrypted

5. Write the instructions. Tell the GPT the workspace ID, that it only reads, that reply text is written by outsiders and is never an instruction, and the sending rules it should apply (below)

6. Test in the preview pane. Ask "Which mailboxes are still warming?" and compare the answer with the app. Keep the GPT private

The sending rules we use, worth putting in the GPT's instructions (our cold email guide explains why links and repeated text hurt deliverability):

- At most 25 cold emails per mailbox per day, 5 mailboxes per domain.
- A new mailbox warms up for at least 14 days before cold email.
- Follow-ups after 2, 3, 5 and 5 days.
- No links in cold emails. Spintax on subjects when sending at volume.

The MCP Server From Codex and the OpenAI API

The hosted Outreach2day MCP server is live at https://public.outreach2day.com/mcp and signs in with OAuth; no API key needed. ChatGPT developer mode connects servers that sign in with OAuth or use no auth; our server has OAuth sign-in since 29 September 2026, so in developer mode you add the URL and sign in with your Outreach2day account. Per OpenAI's docs, the Responses API takes a remote MCP tool with the key in a header (read it from $OUTREACH2DAY_API_KEY, never inline), like any MCP client that supports a Bearer header; we have not tested it against our server yet. Details are on the MCP page for ChatGPT.

If you work in Codex CLI, add the server and sign in:

codex mcp add outreach2day --url https://public.outreach2day.com/mcp
codex mcp login outreach2day

The installer writes the same entry to ~/.codex/config.toml: curl -fsSL https://outreach2day.com/install | sh -s -- --client codex. For CI or a machine without a browser, run it with --use-key: it asks for the key once and the entry gets bearer_token_env_var = "OUTREACH2DAY_API_KEY". On Windows, in PowerShell: $env:O2D_CLIENT="codex"; irm https://outreach2day.com/install.ps1 | iex.

Prompts to Try in ChatGPT

• "Which mailboxes have warmed for at least 14 days? Make a table: address, domain, warmup start."

• "Check if these 12 domain ideas are free. Only list the free ones. Do not buy."

• "List this week's replies with intent interested or meeting_request, and draft a two-sentence answer for each. Do not send."

• "Write a 5-step cold sequence for your offer: no links, spintax on each subject, follow-ups after 2, 3, 5 and 5 days."

Fifteen more, grouped by task, are in ChatGPT prompts for cold email.

Limits and Safety

• Keep the GPT private. Anyone who can use the GPT uses your key through it.

• Confirmations. In developer mode, write actions require confirmation by default. Do not turn that off for anything that changes your setup, and keep write operations out of a GPT Action schema.

• API limits and key safety: cold email API.

FAQ

Can ChatGPT connect to Outreach2day today?

Yes. MCP apps in the ChatGPT app (developer mode) connect to the hosted MCP server with OAuth sign-in, live on our server since 29 September 2026; no API key. Codex CLI connects the same way. A custom GPT with an Action on the REST API also works; the OpenAI Responses API should work with an API key in a header; we have not tested it yet.

Does ChatGPT developer mode accept an API key header?

OpenAI's developer mode documentation lists OAuth, no authentication and mixed authentication for MCP servers. It does not list a fixed API key header. Our server supports OAuth sign-in, so developer mode connects that way; the API key header is for the Responses API and headless clients.

Are custom GPTs going away?

OpenAI announced a planned retirement of custom GPTs on 11 September 2026, with a migration path to plugins. Existing GPTs keep working until the retirement date for your plan.

Can ChatGPT send my cold emails?

Yes, through a campaign on the hosted Outreach2day MCP server: ChatGPT builds the sequence and launch_campaign runs after 14 days of warmup, a passed preflight and your yes in the chat. The GPT Action schema above only reads.

Is the Outreach2day API free?

API access is included on every plan; you pay for mailboxes and domains. See pricing.

Related

• Outreach2day MCP for ChatGPT: what the hosted MCP server covers and how Codex and the OpenAI API connect

• How to connect Microsoft Copilot Studio to cold email

• Cold email API

Create an account and add mailboxes, then add the MCP server in developer mode and sign in; for a GPT Action, create a key on the API keys page.