Fall back to os.Hostname() when SMTP_HELLO_NAME is not set

Using localhost as the EHLO identity is the Go net/smtp default but is
rejected by strict relays such as Google Workspace. os.Hostname() is
the conventional SMTP client fallback and returns the actual machine or
pod name in production environments.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/mailer/mailer.go

@@ -22,6 +22,7 @@ import (
 	"fmt"
 	"net"
 	"net/smtp"
+	"os"
 	"time"
 
 	"github.com/jhillyerd/enmime"
@@ -320,12 +321,17 @@ func (h *sendingHandler) sendMail(ctx context.Context, to []string, msg []byte)
 
 	defer func() { _ = c.Quit() }()
 
-	if h.smtp.HelloName != "" {
-		if err := c.Hello(h.smtp.HelloName); err != nil {
-			return fmt.Errorf("SMTP EHLO error: %w", err)
+	helloName := h.smtp.HelloName
+	if helloName == "" {
+		if helloName, err = os.Hostname(); err != nil {
+			helloName = "localhost"
 		}
 	}
 
+	if err := c.Hello(helloName); err != nil {
+		return fmt.Errorf("SMTP EHLO error: %w", err)
+	}
+
 	if h.smtp.TLSRequired {
 		if err := c.StartTLS(&tls.Config{ServerName: host}); err != nil {
 			return fmt.Errorf("TLS negotiation error: %w", err)