sc-imap-mcp 0.2.0

Security-focused Model Context Protocol server mediating LLM access to IMAP mailboxes under strict, auditable policy.

imap-mcp

A security-focused Model Context Protocol server that mediates LLM access to IMAP mailboxes under strict, auditable policy. Designed for agents that need to read, classify, move, and archive mail — but must be prevented from reading what they shouldn't, moving what they shouldn't, or leaving no trace of what they did.

Status

V1 — all 192 BDD scenarios green, 0 skipped. The server is runnable on stdio and HTTP/SSE transports. Gmail and standard IMAP providers are supported.

Quick start

# Install
pip install sc-imap-mcp
# Or from source:
cd server && pip install -e .

# Create a config directory (see Configuration below)
mkdir -p /etc/imap-mcp/{policies}

# Run on stdio (for Claude Desktop, Cline, etc.)
IMAP_MCP_CONFIG_DIR=/etc/imap-mcp \
IMAP_MCP_CALLER_ID=my-agent \
  imap-mcp --transport stdio

# Run on HTTP (for multi-agent setups)
IMAP_MCP_CONFIG_DIR=/etc/imap-mcp \
  imap-mcp --transport http --host 127.0.0.1 --port 8080

Architecture

┌──────────────────────────┐
│  LLM agent (MCP client)  │
└────────────┬─────────────┘
             │ MCP (stdio or HTTP/SSE)
┌────────────▼─────────────────────────────────────────┐
│  imap-mcp                                            │
│  ┌─────────────────────────────────────────────────┐ │
│  │ Policy Decision Point  — default-deny           │ │
│  │ Redaction / Transparency Layer                  │ │
│  │ Transaction Manager (WAL + saga)                │ │
│  │ IMAP Core · Connection Pool · OAuth2 adapters   │ │
│  │ Pluggable Secret Store                          │ │
│  │ Append-only Audit Log with hash chain           │ │
│  └─────────────────────────────────────────────────┘ │
└─────────┬───────────────────┬────────────────────────┘
          │ IMAP              │ IMAP
┌─────────▼────┐   ┌──────────▼────┐
│ Account A    │   │ Account B     │
│ (e.g. Gmail) │   │ (e.g. Dovecot)│
└──────────────┘   └───────────────┘

Configuration

The server reads its configuration from a directory of YAML files pointed to by IMAP_MCP_CONFIG_DIR.

config/
├── accounts.yaml        # IMAP accounts, secret store, audit, WAL
├── callers.yaml         # MCP callers and their auth
└── policies/
    ├── invoice-agent.yaml
    └── overview-bot.yaml

accounts.yaml

Defines every IMAP account the server may connect to, plus the secret store backend, audit log, and WAL configuration.

accounts:
  - id: company-mail
    provider: imap-standard       # or "google" for Gmail
    host: imap.example.com
    port: 993
    auth:
      type: password              # or "xoauth2"
      secret_ref: secret://accounts/company-mail/password
    token_cache: memory_only      # or "persist_all" (OAuth only)

  - id: gmail-invoices
    provider: google
    host: imap.gmail.com
    port: 993
    auth:
      type: xoauth2
      secret_ref: secret://accounts/gmail-invoices/refresh_token
      oauth_scope: https://mail.google.com/
    token_cache: persist_all

secret_store:
  backend: file_dir               # or "env_var" or "gpg_file"
  path: /etc/imap-mcp/secrets

audit:
  directory: /var/log/imap-mcp/audit
  hot_days: 90                    # days before gzip compression
  warm_days: 275                  # additional days as .gz
  delete_after_days: 365          # total age before deletion
  external_root_hook: /usr/local/bin/audit-root-notify

wal:
  path: /var/lib/imap-mcp/wal.db

Account fields

Field	Required	Default	Description
id	yes	—	Unique identifier referenced in policies
provider	no	imap-standard	imap-standard, google, or google-mock
host	no	127.0.0.1	IMAP server hostname
port	no	143	IMAP port (993 for IMAPS)
auth.type	yes	—	password or xoauth2
auth.secret_ref	yes	—	Reference to the secret store (e.g. secret://accounts/x/password)
auth.oauth_scope	no	—	OAuth2 scope for xoauth2 accounts
token_cache	no	memory_only	memory_only (access tokens in RAM only) or persist_all (also persisted)

Secret store backends

Backend	Description	Config fields
file_dir	Plaintext files; confidentiality from the surrounding system (git-crypt, SOPS, LUKS)	path
env_var	Read-only from environment variables. Secret path segments become UPPER_SNAKE_CASE with __ separators. Read-only — OAuth bootstrap refuses to start against this backend.	—
gpg_file	Per-file GPG decryption using the operator's key	path, recipient, gnupghome

Secret references use the format secret://path/segments. For file_dir, this resolves to {path}/path/segments. For env_var, secret://callers/invoice-agent/token becomes the environment variable IMAP_MCP_SECRET__CALLERS__INVOICE_AGENT__TOKEN.

callers.yaml

Defines every MCP caller (agent) that may connect to the server.

callers:
  - id: invoice-agent
    policy: invoice-policy
    auth:
      type: shared_token
      token_secret_ref: secret://callers/invoice-agent/token

  - id: overview-bot
    policy: overview-policy
    auth:
      type: stdio_trusted

Caller auth types

Type	Transport	Mechanism
stdio_trusted	stdio only	Caller ID set via IMAP_MCP_CALLER_ID env var by the orchestrator. Forbidden on HTTP transport (startup error).
shared_token	stdio + HTTP	Bearer token verified with constant-time comparison. On HTTP: Authorization: Bearer <token> header.

Caller identity is immutable for the session duration. No impersonation primitive exists.

policies/<name>.yaml

Each policy file defines what one caller may see and do.

name: invoice-policy
accounts:
  company-mail:
    - path: INBOX/Invoices
      mode: whitelist
      default: NONE
      mark_seen: true
      move_out: true
      rules:
        - match: { from_domain: hornbach.de }
          grant: FULL
        - match: { from_domain: amazon.de }
          grant: FULL

    - path: Archive/Invoices-2026
      mode: whitelist
      default: NONE
      accept_incoming: true
      rules: []

  gmail-invoices:
    - path: Rechnungen
      mode: whitelist
      default: NONE
      mark_seen: true
      move_out: true
      rules:
        - match: { from_domain: hornbach.de }
          grant: FULL

Folder policy fields

Field	Required	Default	Description
path	yes	—	IMAP folder path (e.g. INBOX, INBOX/Invoices, [Gmail]/All Mail)
mode	yes	—	whitelist (default=NONE, rules grant access) or blacklist (default>NONE, rules cap access)
default	yes	—	Default visibility level when no rule matches
rules	no	[]	Sender-specific overrides (see below)
mark_seen	no	false	Can toggle \Seen flag
mark_tagged	no	false	Can set keywords and \Flagged
move_out	no	false	Can remove messages from this folder
accept_incoming	no	false	Can receive messages moved/copied in
draft_append	no	false	Can append new drafts

Visibility levels

Each rule grants exactly one level from this hierarchy:

NONE < COUNT < METADATA < ENVELOPE < HEADERS < BODY < FULL

Level	What is exposed
NONE	Nothing (message is invisible)
COUNT	Message count only (folder_stats)
METADATA	UIDs, sizes, flags (search)
ENVELOPE	From, To, Subject, Date (fetch_envelope)
HEADERS	Full RFC 5322 header block (fetch_headers)
BODY	Plain-text and HTML bodies (fetch_body)
FULL	Everything including attachments (fetch_attachment)

Sender rule grammar

Rules use a closed set of predicates. Predicates within one rule are AND-combined; multiple rules in a folder are OR-combined.

Predicate	Type	Example
from	exact email	[e-mail]
from_domain	domain (case-insensitive, trailing-dot tolerant)	hornbach.de
to	exact email	[e-mail]
to_contains	substring	team
subject_contains	substring (case-insensitive, NFC-normalized)	rechnung
has_attachment	boolean	true
newer_than	duration	30d
older_than	duration	90d
size_gt	bytes	10000
size_lt	bytes	1500

In whitelist mode, rules use grant: <level>. In blacklist mode, rules use cap: <level>. Mixing both in one folder is a parse-time error.

MCP tool surface

Seventeen tools, each gated on exactly one visibility level or one capability.

Read tools (9)

Tool	Min visibility	Description
list_accounts	—	List visible accounts + hidden_accounts_count
list_folders	COUNT	List visible folders + hidden_folders_count
list_labels	COUNT	Gmail only: list labels with flags
folder_stats	COUNT	Message counts per visibility level
search	METADATA	Search with matched_total / matched_visible / filtered_out
fetch_envelope	ENVELOPE	From, To, Subject, Date + redaction metadata
fetch_headers	HEADERS	Full RFC 5322 headers
fetch_body	BODY	Plain-text and HTML bodies
fetch_attachment	FULL	MIME attachment bytes

Write tools (5)

Tool	Required capability	Description
mark_seen	mark_seen	Toggle \Seen flag
mark_tagged	mark_tagged	Add/remove keywords
move	move_out + accept_incoming	Move message (intra-account: native MOVE; cross-account: saga)
copy	accept_incoming	Copy message to target folder
create_draft	draft_append	Append RFC 5322 draft

Meta tools (3)

Tool	Description
describe_policy	Caller's own policy profile (accounts, folders, capabilities, hidden counts). Never reveals rule patterns or other callers.
get_caller_identity	Resolved caller_id for the current session
get_transaction_status	WAL state of a cross-account move saga

Deliberately absent

delete, expunge, raw_imap_command, fetch_raw_rfc822, cross-account search, MCP resource subscriptions, folder CRUD, policy reload via MCP. See ADR 0018.

Environment variables

Variable	Required	Description
IMAP_MCP_CONFIG_DIR	yes	Path to the configuration directory
IMAP_MCP_CALLER_ID	stdio_trusted only	Caller identity for stdio transport
IMAP_MCP_HTTP_HOST	no	Bind address for HTTP (default: 127.0.0.1)
IMAP_MCP_HTTP_PORT	no	Port for HTTP (default: 0 = ephemeral)
IMAP_MCP_APPEND_TIMEOUT	no	Timeout in seconds for IMAP APPEND
IMAP_MCP_OAUTH_CLIENT_ID	no	OAuth2 client ID (default: built-in)

OAuth2 bootstrap

For accounts with auth.type: xoauth2, run the interactive bootstrap once per account to obtain the refresh token:

imap-mcp-oauth-bootstrap --account gmail-invoices

This opens a browser for the OAuth consent flow. On success, the refresh token is stored in the secret store. The server then exchanges it for access tokens automatically.

PKCE is mandatory. Scope minimization is enforced: a readonly-scope account refuses write operations regardless of policy.

Policy reload

Send SIGHUP to the running server process. The server re-parses the entire config directory, validates it, and swaps the in-memory state atomically. Parse or validation errors preserve the previous policy and write an audit record with the error. In-flight saga transactions complete under their original policy; new requests see the new one.

kill -HUP $(pidof imap-mcp)

Gmail support

Accounts with provider: google get explicit Gmail semantics:

• list_labels tool available (Gmail only)
• search results include canonical_all_mail_uid for cross-label deduplication
• Intra-account move implemented as label swap (not physical MOVE)
• Cross-account sagas fetch deterministically from [Gmail]/All Mail
• [Gmail]/Trash, [Gmail]/Drafts etc. are policy-addressable folders

Audit log

Append-only JSONL with SHA-256 hash chain, one file per UTC day. Strict no-content-leak rule: no message bodies, subjects, attachment filenames, OAuth tokens, or cleartext sender addresses in DENY records.

/var/log/imap-mcp/audit/
├── 2026-05-06.jsonl       # hot (plain, mode 0600)
├── 2026-02-01.jsonl.gz    # warm (gzipped, mode 0400)
└── ...

Each record contains: ts, seq, prev_hash, tool, caller_id, decision (ALLOW/DENY), reason, result, and tool-specific fields.

Implementation stack

• Language: Python 3.11+
• IMAP: aioimaplib
• MCP: the official mcp SDK (stdio + HTTP/SSE)
• Storage: aiosqlite for WAL; YAML for config
• Validation: pydantic v2 (strict mode)
• OAuth: httpx + first-party flow implementation
• Testing: pytest (property tests) + behave (BDD, 192 scenarios)

Testing

# BDD suite (requires Docker for IMAP fixtures)
cd bdd && docker compose -f docker/docker-compose.yml up -d
.venv/bin/behave --no-color --format=progress features/

# Server property tests
cd server && .venv/bin/pytest tests/policy/ -q

Documentation

The design is captured in 23 ADRs under docs/adr/. Limitation records under docs/limitations/ (all 9 resolved). Error-path analysis at docs/error_path_analysis.md.

License

GPL-3.0-or-later. See LICENSE.