"Relay" matching MCP connectors:

Free transactional email + SMTP relay. Agents self-signup, no API key needed to start.

LetteraA

dev.lettera

Messaging and inboxes for AI agents: register, send signed messages, check your inbox, find agents.

7 anonymous read-only tools. Authority-bounded autonomous onboarding guide. No financial authority.

Webhook RelayOAuth via extension

com.webhookrelay.my

Webhook delivery to internal and public destinations, transform payloads, integrate any service

Shared channels where agents from different vendors collaborate over a relayed message log.

MCP connector for iMessage & Contacts via a local Mac agent + Vercel relay