Install
One address, whatever assistant you use. You sign in with your ordinary Postern Mail account, enable every function, and let your mailbox switches decide the rest. About three minutes.
Before you start
Create a Postern Mail account at app.posternmail.ai, add at least one mailbox, and tick the switches you want on it. Two is a good start: read messages and draft replies. You can change switches at any time; the assistant sees the change on its next call, without reconnecting.
Your mailboxes and their switches live in the Postern Mail app only. The assistant can list them but can never change them.
The basics
01
Every client gets the same one:
Transport is Streamable HTTP. Do not add /sse or any path.
02
Almost every client registers itself with Postern Mail automatically. Leave the OAuth client id and client secret fields empty.
If your client asks you to name the connection anyway, use any name you like — "Claude on my laptop" is fine. There is no client secret; Postern Mail never issues one.
03
The client opens your browser on the Postern Mail sign-in page. Sign in with your normal account, approve the connection, and you are back in the assistant.
No separate MCP password, token or API key exists. Every approved connection appears under Account → Connected assistants, where you can revoke it in one click.
04
Switch on every Postern Mail function in your assistant, and allow them to run without asking you each time.
That is safe by design — see below.
Why "enable everything" is the safe choice
Your assistant only holds a key to Postern Mail. Every call it makes is checked on our server against the switches you set per mailbox, freshly, before anything happens. A function you did not allow for a mailbox is refused there, with the reason, no matter what the assistant's own settings say.
Per-call approval prompts in the assistant therefore add friction, not protection. Turn them off, enable all thirteen functions, and set your real limits in Postern Mail — where they are enforced, and where switching one off takes effect on the very next call.
Per client
Claude and ChatGPT connect from their own servers, so the address must be the public one above. Claude Code, Cursor and VS Code run on your machine and use the same address.
Custom connectors work on Free, Pro, Max, Team and Enterprise plans (Free allows one connector).
https://mcp.posternmail.ai. Leave the advanced OAuth fields empty — Claude registers itself.Team and Enterprise: an owner adds the connector once under Organization settings → Connectors; each member then signs in individually under Customize → Connectors. Local MCP servers configured in claude_desktop_config.json are a different mechanism — use the connector flow above.
claude mcp add --transport http multimail https://mcp.posternmail.ai--scope user to have it in every project, or --scope project to share it with your team through a committed .mcp.json./mcp, choose multimail and Authenticate. Your browser opens the Postern Mail sign-in and approval screen; approve and return to the terminal.The server's machine name is multimail, Postern Mail's internal codename. Keep it; it appears in configuration keys.
Custom MCP connectors need a Plus, Pro, Business, Enterprise or Edu plan and live behind developer mode.
https://mcp.posternmail.ai. Authentication: OAuth. Leave client id and secret empty — ChatGPT registers itself..cursor/mcp.json, global configuration in ~/.cursor/mcp.json.{
"mcpServers": {
"multimail": {
"url": "https://mcp.posternmail.ai"
}
}
}.vscode/mcp.json in the workspace, or to your user-level mcp.json:{
"servers": {
"multimail": {
"type": "http",
"url": "https://mcp.posternmail.ai"
}
}
}Most clients use the same shape as Cursor: point them at the address with Streamable HTTP transport.
{
"mcpServers": {
"multimail": {
"url": "https://mcp.posternmail.ai"
}
}
}Some older clients cannot talk to a remote server directly. Bridge with mcp-remote:
{
"mcpServers": {
"multimail": {
"command": "npx",
"args": ["-y", "mcp-remote", "https://mcp.posternmail.ai"]
}
}
}Verify
Ask your assistant: "Use the Postern Mail ping tool." It should answer pong — Postern Mail MCP server is up.
Then: "Which mailboxes do I have?" It should list your mailboxes by name, with the switches you turned on. From here, just ask — by mailbox name, in ordinary language.
Troubleshooting
| Symptom | Cause and fix |
|---|---|
| Authentication error on every call | The connection was never approved, was revoked, or the account is closed. Re-run the sign-in from your client and check Account → Connected assistants. |
| "This connection request is no longer valid" | The sign-in link expired (about ten minutes) or was reused. Start again from the assistant. |
| Connector added, no functions appear | Enable the connector in the specific conversation (Claude: + → Connectors). |
| A function is refused | The switch for that mailbox is off. The refusal names it; turn it on in Postern Mail and ask again — no reconnect needed. |
Client insists on an /sse address | It uses the retired HTTP+SSE transport. Use a client with Streamable HTTP support, or bridge with mcp-remote. |
| Claude or ChatGPT cannot reach the server | You used a localhost address in a cloud client. Use https://mcp.posternmail.ai. |