Allow separate SMTP TLS server name

README.md

@@ -22,6 +22,7 @@ Recommended settings:
 - `SMTP_PORT=587`
 - `SMTP_STARTTLS=true`
 - `SMTP_HELO_NAME=mailer.example.invalid`
+- `SMTP_TLS_SERVER_NAME=smtp.example.invalid`
 - `SMTP_USERNAME`
 - `SMTP_PASSWORD`
 - `PRINT_REPORT=true`

main.go

@@ -40,6 +40,7 @@ type Config struct {
 	SMTPPassword   string
 	SMTPStartTLS   bool
 	SMTPHeloName   string
+	SMTPTLSServer  string
 	EmailFrom      string
 	EmailTo        []string
 	PrintReport    bool
@@ -177,6 +178,10 @@ func loadConfig() (Config, error) {
 	if err != nil {
 		return Config{}, err
 	}
+	smtpTLSServer, err := readValue("SMTP_TLS_SERVER_NAME", "", false)
+	if err != nil {
+		return Config{}, err
+	}
 	emailFrom, err := readValue("EMAIL_FROM", "", false)
 	if err != nil {
 		return Config{}, err
@@ -210,6 +215,7 @@ func loadConfig() (Config, error) {
 		SMTPPassword:   smtpPassword,
 		SMTPStartTLS:   smtpStartTLS,
 		SMTPHeloName:   normalizeSMTPHeloName(smtpHeloName),
+		SMTPTLSServer:  strings.TrimSpace(smtpTLSServer),
 		EmailFrom:      emailFrom,
 		EmailTo:        emailTo,
 		PrintReport:    printReport,
@@ -601,12 +607,13 @@ func sendEmail(cfg Config, subject, body string) error {
 
 	host := cfg.SMTPHost
 	heloName := normalizeSMTPHeloName(cfg.SMTPHeloName)
+	tlsServerName := firstNonEmpty(cfg.SMTPTLSServer, host)
 	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 {
+			if err := conn.StartTLS(&tls.Config{ServerName: tlsServerName, MinVersion: tls.VersionTLS12}); err != nil {
 				return err
 			}
 			if err := conn.Hello(heloName); err != nil {