Maddy - "implements all functionality required to run a e-mail server. It replaces Postfix, Dovecot, OpenDKIM, OpenSPF, OpenDMARC and more with one daemon." written in Go. last release: 0.8.2, January 2026.
I didn't find documentation for deploying #Maddy as a #Chatmail relay, but I also only looked for like 2 minutes.
Mox - "For sending and receiving email. With support for IMAP4, SMTP, SPF, DKIM, DMARC, MTA-STS, DANE and DNSSEC, reputation-based and content-based junk filtering, Internationalization (EIA/IDNA), automatic TLS with ACME and Let's Encrypt, account autoconfiguration, webmail." written in Go. last release: 0.0.15, April 2025.
Appears to have a #Chatmail mode.
relevant: https://github.com/mjl-/mox/issues/251