diff --git a/proxy_server.py b/proxy_server.py
index 7a4a583..e37b23a 100644
--- a/proxy_server.py
+++ b/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()
 
diff --git a/tests/test_proxy_server.py b/tests/test_proxy_server.py
index d01ec91..c904ac9 100644
--- a/tests/test_proxy_server.py
+++ b/tests/test_proxy_server.py
@@ -126,14 +126,17 @@ def test_smtp_proxy_handler_forwards_message_over_ssl(monkeypatch):
     Settings.BACKEND_SMTP_USER = "smtp-user"
     Settings.BACKEND_SMTP_PASS = "smtp-pass"
 
+    # Include an 8-bit byte that would be mangled by a utf-8 decode.
+    raw = b"Subject: Test\r\n\r\nBody \x80\r\n"
     handler = SMTPProxyHandler()
-    handler.send_message("from@example.com", ["to@example.com"], b"Subject: Test\r\n\r\nBody\r\n")
+    handler.send_message("from@example.com", ["to@example.com"], raw)
 
     assert captured["instance"].host == Settings.BACKEND_SMTP_HOST
     assert captured["instance"].logged_in is True
     assert captured["instance"].sent[0] == "from@example.com"
     assert captured["instance"].sent[1] == ["to@example.com"]
-    assert "Subject: Test" in captured["instance"].sent[2]
+    # Raw bytes must be forwarded unchanged, not decoded.
+    assert captured["instance"].sent[2] == raw
 
     Settings.BACKEND_SMTP_USE_SSL = previous_ssl
     Settings.BACKEND_SMTP_USER = previous_user