diff --git a/proxy_server.py b/proxy_server.py
index 77e7a27..c599546 100644
--- a/proxy_server.py
+++ b/proxy_server.py
@@ -344,7 +344,7 @@ class SMTPProxyHandler:
         await asyncio.to_thread(self.send_message, envelope.mail_from, envelope.rcpt_tos, envelope.content)
         return "250 Message accepted for delivery"
 
-        def send_message(self, sender, recipients, data):
+    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:
diff --git a/requirements.txt b/requirements.txt
index 595c94e..0bc8eef 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,2 +1,2 @@
-aiosmtpd>=1.6.3
+aiosmtpd>=1.4.6,<1.5
 pytest>=8.0.0
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..ccb70b5
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,7 @@
+import os
+import sys
+
+# Ensure the project root is on sys.path when running tests.
+ROOT = os.path.dirname(os.path.dirname(__file__))
+if ROOT not in sys.path:
+    sys.path.insert(0, ROOT)
diff --git a/tests/test_proxy_server.py b/tests/test_proxy_server.py
index 02f349b..bb2cfab 100644
--- a/tests/test_proxy_server.py
+++ b/tests/test_proxy_server.py
@@ -114,7 +114,7 @@ def test_smtp_proxy_handler_forwards_message_over_ssl(monkeypatch):
     assert captured["instance"].logged_in is True
     assert captured["instance"].sent[0] == "from@example.com"
     assert captured["instance"].sent[1] == ["to@example.com"]
-    assert b"Subject: Test" in captured["instance"].sent[2]
+    assert "Subject: Test" in captured["instance"].sent[2]
 
     Settings.BACKEND_SMTP_USE_SSL = previous_ssl
     Settings.BACKEND_SMTP_USER = previous_user