Proton Bridge MCP

CLI and MCP server for sending correctly threaded replies through Proton Mail Bridge.

The main lesson encoded here is simple:

• Use Proton Bridge SMTP with explicit In-Reply-To and References for agent-sent replies.

• Do not create same-thread reply drafts by IMAP APPEND; Proton Bridge may rewrite those draft headers and the sent email can leave the original conversation.

• Proton web/app reply composer is also safe because it creates the reply from the existing conversation.

Features

• Search message headers through Proton Bridge IMAP.

• Find a thread anchor by sender and subject.

• Send a threaded reply through Proton Bridge SMTP.

• Verify sent-message headers.

• Expose the same operations as MCP tools for coding agents.

• Sending tools are dry-run by default unless confirm_send is set.

Related MCP server: ProtonMail MCP Server

Requirements

• Python 3.10+

• Proton Mail Bridge running locally

• Bridge IMAP enabled, usually 127.0.0.1:1143

• Bridge SMTP enabled, usually 127.0.0.1:1025

No third-party Python dependencies are required.

Configuration

Set the Bridge account and Bridge mailbox password:

export PROTON_BRIDGE_ACCOUNT="you@example.com"
export PROTON_BRIDGE_PASSWORD="your-bridge-mailbox-password"

PROTON_PW is also accepted for the password if you already use that name.

These are Proton Bridge credentials, not necessarily your Proton web password.

CLI Usage

From the repo root:

python3 -m krantiutils_proton.cli search \
  --from-addr collaborator@example.com \
  --subject-contains "Paper draft" \
  --limit 3

Dry-run a threaded reply to the latest matching incoming message:

python3 -m krantiutils_proton.cli reply-latest \
  --thread-from collaborator@example.com \
  --subject-contains "Paper draft" \
  --to collaborator@example.com \
  --body-file /tmp/reply.txt \
  --attach /tmp/paper.pdf

Actually send only after checking the dry-run output:

python3 -m krantiutils_proton.cli reply-latest \
  --thread-from collaborator@example.com \
  --subject-contains "Paper draft" \
  --to collaborator@example.com \
  --body-file /tmp/reply.txt \
  --attach /tmp/paper.pdf \
  --confirm-send

Verify sent headers:

python3 -m krantiutils_proton.cli verify-sent \
  --to-addr collaborator@example.com \
  --subject-contains "Paper draft" \
  --limit 5

Send a controlled threading test to yourself:

python3 -m krantiutils_proton.cli thread-test \
  --to you+thread-test@example.com \
  --count 3 \
  --confirm-send

MCP Server

Run:

./proton-bridge-mcp

Codex-style config:

[mcp_servers.proton_bridge]
command = "/absolute/path/to/proton-bridge-mcp/proton-bridge-mcp"
env = { PROTON_BRIDGE_ACCOUNT = "you@example.com" }

Set PROTON_BRIDGE_PASSWORD in the environment available to the MCP process.

Available tools:

• proton_search_headers

• proton_latest_thread_anchor

• proton_send_email

• proton_reply_to_latest

• proton_verify_sent

• proton_send_thread_test

Agent Workflow

For a thread-safe reply:

1. Use proton_latest_thread_anchor to find the latest incoming message in the target thread.

2. Inspect the returned message_id, in_reply_to, references, sender, and subject.

3. Dry-run proton_reply_to_latest without confirm_send.

4. Check recipient, subject, attachment paths, and the generated In-Reply-To/References.

5. Send only after explicit user approval with confirm_send: true.

6. Use proton_verify_sent to confirm the sent copy kept thread headers.

Never claim an IMAP-created draft is definitely on the original thread unless the stored draft has the expected In-Reply-To and References headers.