MCP for Thunderbird

MCP for Thunderbird

MCP for Thunderbird lets AI apps on your computer work with the mail in Thunderbird, within limits you set in Thunderbird.

AI apps such as Claude Desktop, Claude Code, Codex and Cursor connect to outside tools through the Model Context Protocol (MCP), an open standard. MCP for Thunderbird gives them a set of mail tools: search your mail, read a message, tidy your inbox, write a draft for you to check. You choose which accounts they may use and what they may do, and you can pause them at any time from Thunderbird's toolbar.

MCP for Thunderbird works with Thunderbird. It is not made by, endorsed by or affiliated with Mozilla or MZLA Technologies, the makers of Thunderbird.

How it works

MCP for Thunderbird has two parts:

• The add-on, installed in Thunderbird. It does the mail work through Thunderbird's standard add-on interfaces, and it holds your settings, the activity log and the pause switch.
• The host, a small program installed on the same computer. Thunderbird starts it when Thunderbird starts, and it stops when Thunderbird closes. AI apps talk to the host, and the host passes their requests to the add-on.

Thunderbird already has your mailbox logins and a copy of your mail, so MCP for Thunderbird never asks for a mail password. Nothing goes through any outside server: the add-on, the host and the AI apps talk to each other only on your computer. The AI app itself then sends what it reads to its vendor, as it does with anything else you give it. PRIVACY.md sets out exactly what goes where.

What AI apps can do

The tools come in groups, and you switch each group on or off in the add-on's options. A tool in a group that is off is not offered to AI apps at all.

AI apps write the body of a message as plain text, and the add-on turns it into an HTML message: a blank line starts a new paragraph, a paragraph of lines that start with - or * becomes a bulleted list, and a paragraph of lines that start with 1. , 2. and so on becomes a numbered list. An indented line continues the list item above it. A list needs a blank line between it and the text around it; otherwise its lines stay as typed.

An AI app can send the body as HTML instead, with html set to true, when it needs tables or formatting. The add-on keeps paragraphs, line breaks, bulleted and numbered lists, bold and italic, links to web and mail addresses, and tables, and gives each table a border and cell padding so it reads the same in Thunderbird and in the recipient's mail program. Everything else is removed: scripts, styles, images and any other markup. The identity's signature and, in a reply or forward, the quoted original still follow the text.

A reply or forward goes out from the identity the original was addressed to, as in Thunderbird: one of its own account's identities first, then one of any other enabled account; for mail you sent, from the identity that sent it. An AI app can name another identity, by its address, with from: for a reply or forward, an identity of any enabled account; for a new message or a direct send, one of the account's identities. A draft written from another account's identity is saved in that account's Drafts folder. With account, a reply or forward is written from that account, and saved in its Drafts folder, even when the original is in another account. A reply can go to a mailing list (kind reply_list) when the original names one in its List-Post header, and a forward can attach the whole original message (forward_as set to attachment) instead of quoting it. mail_compose returns the ids of the compose tab and window it opened.

What it cannot do

• Delete mail for good. The organise tools can move mail to Trash, but no tool empties Trash or Junk, deletes a folder or changes account settings.
• Edit Thunderbird's message filters. Thunderbird gives add-ons no way to change filters, so mail_filter_suggest writes out the steps for you to follow in Thunderbird's Message Filters window.
• Calendar and tasks. Thunderbird has no stable add-on interface for them.
• Work with AI apps in a web browser, such as ChatGPT or claude.ai in a browser tab. Those reach tools through a server on the internet, and MCP for Thunderbird only listens on your own computer.
• Work when Thunderbird is closed. The host runs only while Thunderbird runs. An AI app that asks in the meantime is told "Thunderbird is not running".
• Run on Thunderbird for Android.

Safety defaults

• No accounts until you tick them. On first run a setup page asks which accounts AI apps may use. None is ticked to begin with.
• "Read and organise" is the suggested level. Drafting and contacts are off until you turn them on.
• No sending. With the default settings, nothing an AI app does can send mail or delete anything for good. Drafts and the compose window leave the decision to send with you. Direct sending stays off unless you turn it on in the options, list the addresses or domains it may send to, and allow it in Thunderbird's own permission prompt.
• Search never returns message text. Search results are pointers: sender, subject, date, folder, flags. An AI app reads the text of a message only by asking for that one message.
• Message text is marked as untrusted. Hidden text, scripts and invisible characters are removed as far as practical, and what remains is wrapped in a block that tells the AI app it is data from the sender, never instructions.
• Local only, with an access key. The host listens only on 127.0.0.1 and answers only requests that carry its access key. The AI apps that setup connects read the key by themselves, so you never copy it by hand.
• Excluded folders stay excluded. A folder you exclude in the options stays hidden from AI apps when it is renamed or moved in Thunderbird, and in an account with excluded folders a new folder, which may be one renamed in webmail, stays hidden until you check the excluded folders; SECURITY.md has the details.
• You can see and stop it. The toolbar button shows whether MCP for Thunderbird is connected, paused or not connected, whether its host program is missing or could not be started, and when an excluded folder was not found or a new folder is hidden, and its menu has a Pause button that refuses every request until you resume. The activity log in the options lists the last 500 requests (time, app, tool and outcome), without message text or addresses.

SECURITY.md describes the threats and the protections in detail, including their limits.

Supported versions

• Thunderbird 140 ESR or later. Versions before 140 are not supported.
• Only Linux is supported for the moment. The Windows and macOS packages are built and tested in CI, but have not yet been checked by hand on a real PC or Mac; they are on the releases page for anyone who wants to try them, and support for them follows once that check is done.
• On Thunderbird 140 ESR, mail_draft and mail_send are not offered: the interface that sends a message without opening a window came in Thunderbird 153, and drafts are offered from 153 too, the versions they are tested on. mail_compose still opens a filled-in compose window for you to check and send, and every other tool works as on later versions. AI apps do not see them in the tool list there, and a call to either by name fails as an unknown tool.
• On Thunderbird 140 ESR, the text of a text attachment (mail_attachment on a text/plain or text/html attachment) is right only where its characters are in Latin-1: Thunderbird 140 hands add-ons the text it decoded itself with each character cut to one byte, so a character outside Latin-1, such as €, comes out as another character. Thunderbird 153 and later hand over the attachment's own bytes, which are decoded in the charset its headers name. Saving an attachment is not affected.
• Tested on Linux with Thunderbird 140.17.0esr, 153.5.1esr and 159.0: every end-to-end test passed on each, run locally on 1 November 2026. The end-to-end tests added on 3 November 2026 (search arguments and threads, attachment types and headers, accounts and folders, excluded folders that follow renames, tags, contacts, archiving into a new year folder, reply identities, attachment placeholders in forwards, offline sending and the host start messages) passed on 153.6.0esr, and on 4 November 2026 the whole suite passed locally on 140.17.0esr and 159.0 as well. In CI, the e2e-linux workflow runs the full end-to-end suite on 140 ESR and on the current release. On Windows and macOS, CI installs the current release and runs the reading and organising end-to-end tests (TestE2ERead and TestE2EOrganise); the other end-to-end tests (among them folder search, search arguments and threads, drafts and sending, the injection defaults, connect mode and the toolbar) run on Linux only. docs/PLATFORMS.md lists what runs where.
• Account types: tested with IMAP. MCP for Thunderbird uses only Thunderbird's general mail interfaces, so other account types Thunderbird supports, such as Gmail and Microsoft 365 over IMAP, should behave the same. Exchange accounts added with Thunderbird's built-in Exchange support have not been tested.
• Linux: Thunderbird downloaded from thunderbird.net, and distribution packages that are not a Snap or Flatpak, work. The Snap (what apt install thunderbird gives you on current Ubuntu) has not been tested, and the Flatpak is not supported; see INSTALL.md.

Quick start

1. Install the host for your system from the GitHub releases page; only Linux is supported for the moment (see Supported versions). On Linux, then run mcp-for-thunderbird setup as your own user. Windows and macOS warn that it comes from an unknown publisher; INSTALL.md shows how to continue.
2. Install the add-on in Thunderbird from the same releases page: download mcp-for-thunderbird- .xpi, open the ≡ menu, choose Add-ons and Themes, then the gear button, then Install Add-on From File…, and pick the file. (A listing on addons.thunderbird.net, which updates the add-on by itself, is to follow.)
3. The add-on opens its setup page: tick the accounts AI apps may use, choose what they may do, and tick the AI apps to set up.
4. Restart your AI app and ask it, for example, "What needs my attention in my inbox today?"

INSTALL.md has the ful