Open-source Email Outbound tools

The best open-source Email Outbound tools, curated: 11 in the registry, including proton-bridge, Cold Outbound Skills, offer-builder. Every tool is open-source; each entry lists license, self-hostability, and links.

11 tools · ← all categories · JSON

• proton-bridge · tool, 2026-10-13
  Proton Bridge is Proton's official, GPL-3.0 desktop app that runs a local IMAP/SMTP server so standard mail clients (Outlook, Thunderbird, Apple Mail) can send and receive end-to-end-encrypted Proton Mail, decrypting locally after Proton login. Written in Go; it is the sanctioned integration path, so no terms-of-service risk.
• Cold Outbound Skills · tool, 2026-09-26
  Thirty open-source Claude Code skills covering the cold-email motion end to end — lead sourcing, copywriting, deliverability/infrastructure setup and campaign operations — packaged by growth operator Eric Nowoslawski. MIT-licensed; distinct from the Corey Haines marketingskills collection.
• offer-builder · tool, 2026-09-12
  Open-source Claude Code skill (MIT) that turns dense market material — industry reports, transcripts, research — into a cold-email-ready offer: it identifies the real problem and its cost, then produces an Offer Sheet with market analysis, value proposition, elevator pitch and a targeting matrix for list building. Deliberately stops before copywriting.
• Emailclaw · tool, 2026-09-09
  Open-source email AI agent (Java) that uses the email subject line as a project boundary to plan and run multi-step tasks with strict per-workspace isolation — an inbox-native automation surface. VERY EARLY: single-digit stars.
• gmailctl · tool, 2026-09-03
  ~2200 stars; complements google-api-python-client; Uses official Gmail API to sync filters. Auth via OAuth2. Diff-before-apply. No ToS risk.
• google-api-python-client (Docs/Drive) · tool, 2026-09-03
  ~8900 stars; complements gmailctl; Official Google API client (maintenance mode but supported). Gmail API auth via OAuth2. No ToS risk; per-user API quotas.
• himalaya · tool, 2026-09-03
  ~7000 stars; complements imap_tools, isync/mbsync; Open protocols (IMAP/SMTP/JMAP) plus Gmail REST and Graph backends. Auth: password/OAuth2/XOAUTH2. No ToS risk.
• imap_tools · tool, 2026-09-03
  ~836 stars; complements himalaya; Open IMAP protocol (over imaplib). Auth via credentials/app-password/OAuth. No ToS risk.
• mail-parser (SpamScope) · tool, 2026-09-03
  ~452 stars; complements email-reply-parser; Pure parsing library (no network/auth). RFC-compliant, Outlook .msg support, defect detection. No ToS risk.
• msgraph-sdk-python (Microsoft Graph) · tool, 2026-09-03
  ~624 stars; complements python-o365, exchangelib; Official Microsoft Graph v1.0 Python SDK. Auth via MSAL OAuth2 (app/delegated). No ToS risk.
• python-o365 · tool, 2026-09-03
  ~1900 stars; complements msgraph-sdk-python, exchangelib; Ergonomic wrapper over Microsoft Graph. Auth via OAuth2. No ToS risk; friendlier than raw Graph SDK.