There are 2 major go written ones (aka the backend mailserver parts - no frontend) GitHub - gopistolet/gopistolet: Mailserver written in Go and GitHub - bradfitz/go-smtpd: SMTP server library for Go. go-smtpd is mostly just a smtp library, but gopistolet is currently written to be a complete webserver.
Just out of curiousity does anyone know a good server relay (a go written binary) listening on mails pushed to localhost and send with authentication and encryption to an smtp server? I already use the usual linux programs, but watching go alternatives is always nice.