Fix.

cmd/smtpd/main.go

@@ -7,6 +7,11 @@ import (
 	"log"
 )
 
+func authenticate(peer smtpd.Peer, username, password string) error {
+	log.Printf("Auth: %s / %s", username, password)
+	return nil
+}
+
 func dumpMessage(peer smtpd.Peer, env smtpd.Envelope) error {
 	log.Printf("New mail from: %s", env.Sender)
 	return nil
@@ -33,6 +38,7 @@ func main() {
 
 	server := &smtpd.Server{
 		Handler:        dumpMessage,
+		Authenticator:  authenticate,
 		TLSConfig:      tlsConfig,
 		ForceTLS:       true,
 	}

smtpd.go

@@ -249,7 +249,7 @@ func (session *session) extensions() []string {
 		extensions = append(extensions, "STARTTLS")
 	}
 
-	if session.tls {
+	if session.server.Authenticator != nil && session.tls {
 		extensions = append(extensions, "AUTH PLAIN LOGIN")
 	}