fix: relay raw SMTP bytes without decoding

send_message decoded the message body with utf-8/errors="replace",
corrupting 8-bit content before forwarding. Pass the raw bytes straight
to smtp.sendmail so the message is relayed unchanged.

Fixes #4

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

proxy_server.py

@@ -416,7 +416,6 @@ class SMTPProxyHandler:
 
     def send_message(self, sender, recipients, data):
         """Forward a complete SMTP message to the backend SMTP server."""
-        message = data.decode("utf-8", errors="replace")
         if Settings.BACKEND_SMTP_USE_SSL:
             smtp = smtplib.SMTP_SSL(Settings.BACKEND_SMTP_HOST, Settings.BACKEND_SMTP_PORT, timeout=30)
         else:
@@ -426,7 +425,7 @@ class SMTPProxyHandler:
         try:
             if Settings.BACKEND_SMTP_USER and Settings.BACKEND_SMTP_PASS:
                 smtp.login(Settings.BACKEND_SMTP_USER, Settings.BACKEND_SMTP_PASS)
-            smtp.sendmail(sender, recipients, message)
+            smtp.sendmail(sender, recipients, data)
         finally:
             smtp.quit()