ThunderSend MCP

A compose-only MCP server for Thunderbird and Betterbird. An AI assistant connected through it can do exactly two things with your mailbox:

• saveDraft -- write a message to the Drafts folder.

• sendMail -- open a compose window for you to review and send (or, only if you explicitly disable the safety setting, send directly).

Two helper tools exist so a client can pick a sender identity: listAccounts and getAccountAccess. That is the whole surface. There is no tool to read, search, move, tag, delete, forward or reply to mail, and no contact, calendar, folder or filter access. The code for those features was removed from the extension, not merely hidden behind a flag.

ThunderSend is a minimal fork of TKasperczyk/thunderbird-mcp (MIT). The compose, draft, attachment-safety and authentication code is taken verbatim from upstream v0.9.0; see LICENSE.

How it works

MCP client (Claude, etc.)  <-- stdio -->  mcp-bridge.cjs  <-- HTTP 127.0.0.1 -->  ThunderSend extension

• The extension runs an HTTP server bound to 127.0.0.1 on port 8765 (or the next free port). Every request needs a Bearer token that is generated randomly on each start with the XPCOM random generator.

• Port and token are written to <temp>/thundersend-mcp/connection.json with mode 0600 and O_EXCL; the bridge reads that file. Nothing is sent anywhere else.

• The bridge and the extension each enforce a hard allow-list of the four tools.

• The manifest has no update_url: the add-on never fetches updates from a third-party server. Rebuild and reinstall to upgrade.

Related MCP server: Thunderbird MCP

Safety defaults

Attachments are subject to the same deny-list as upstream: credential stores, SSH/GPG/cloud keys, browser and Thunderbird profile files, symlinks and junctions are refused on both the bridge and the extension side.

Install

1. Build the XPI:

   node scripts/build-xpi.cjs

2. In Thunderbird/Betterbird: Add-ons and Themes > gear icon > Install Add-on From File, pick dist/thundersend-mcp.xpi, then restart.

3. Add the bridge to your MCP client:

   {
     "mcpServers": {
       "thundersend": {
         "command": "node",
         "args": ["C:\\git-local-26-06\\thundersend-mcp\\mcp-bridge.cjs"]
       }
     }
   }

Set THUNDERBIRD_MCP_CONNECTION_FILE if the bridge cannot find connection.json automatically.

ThunderSend uses its own add-on id, preference branch and temp directory, so it can be installed next to the full thunderbird-mcp add-on without the two interfering.

Tools

saveDraft

to, subject, body, cc, bcc, isHtml, from, attachments, inReplyTo, references, replaceMessageId + replaceFolderPath, includeSignature. All optional. Writes to the selected identity's Drafts folder and returns its URI.

sendMail

to, subject, body required; cc, bcc, isHtml, from, attachments, skipReview, includeSignature optional. With the default settings this opens a compose window and returns immediately; the human presses Send.

listAccounts / getAccountAccess

Read-only lists of accounts and identities the server is allowed to send from.

Development

npm install
npm run lint
npm test
npm run build

Tests load production code by // BEGIN ... // END markers and run it in a VM with Thunderbird services mocked, exactly as upstream does.