Fix SMTP submission HELO handling

main.go

@@ -39,6 +39,7 @@ type Config struct {
 	SMTPUsername   string
 	SMTPPassword   string
 	SMTPStartTLS   bool
+	SMTPHeloName   string
 	EmailFrom      string
 	EmailTo        []string
 	PrintReport    bool
@@ -172,6 +173,10 @@ func loadConfig() (Config, error) {
 		return Config{}, err
 	}
 	smtpStartTLS := readBool("SMTP_STARTTLS", true)
+	smtpHeloName, err := readValue("SMTP_HELO_NAME", "", false)
+	if err != nil {
+		return Config{}, err
+	}
 	emailFrom, err := readValue("EMAIL_FROM", "", false)
 	if err != nil {
 		return Config{}, err
@@ -204,6 +209,7 @@ func loadConfig() (Config, error) {
 		SMTPUsername:   smtpUsername,
 		SMTPPassword:   smtpPassword,
 		SMTPStartTLS:   smtpStartTLS,
+		SMTPHeloName:   normalizeSMTPHeloName(smtpHeloName),
 		EmailFrom:      emailFrom,
 		EmailTo:        emailTo,
 		PrintReport:    printReport,
@@ -594,11 +600,18 @@ func sendEmail(cfg Config, subject, body string) error {
 	defer conn.Close()
 
 	host := cfg.SMTPHost
+	heloName := normalizeSMTPHeloName(cfg.SMTPHeloName)
+	if err := conn.Hello(heloName); err != nil {
+		return err
+	}
 	if cfg.SMTPStartTLS {
 		if ok, _ := conn.Extension("STARTTLS"); ok {
 			if err := conn.StartTLS(&tls.Config{ServerName: host, MinVersion: tls.VersionTLS12}); err != nil {
 				return err
 			}
+			if err := conn.Hello(heloName); err != nil {
+				return err
+			}
 		}
 	}
 	if cfg.SMTPUsername != "" {
@@ -641,6 +654,23 @@ func sendEmail(cfg Config, subject, body string) error {
 	return conn.Quit()
 }
 
+func normalizeSMTPHeloName(value string) string {
+	value = strings.TrimSpace(value)
+	if value == "" {
+		hostname, err := os.Hostname()
+		if err == nil {
+			value = strings.TrimSpace(hostname)
+		}
+	}
+	if value == "" {
+		return "localhost.localdomain"
+	}
+	if strings.Contains(value, ".") {
+		return value
+	}
+	return value + ".localdomain"
+}
+
 func inferAssignmentDate(monthDay string, now time.Time) time.Time {
 	return inferDate(monthDay, now, false)
 }