axllent / mailpit

An email and SMTP testing tool with API for developers

decke / smtprelay

Simple Golang SMTP relay/proxy server

KumoCorp / kumomta

The first Open-Source high-performance MTA developed from the ground-up for high-volume email sending environments.

truespar / sentio

Email inbox API for AI agents. Give every agent its own real email address, receive mail as structured webhooks, and reply in-thread over REST. A complete multi-tenant mail server in Rust: inbound and outbound, DKIM/SPF/DMARC/ARC, MTA-STS, DANE, three-tier anti-spam.