feat: add unit test for mail sending feature

Author: corp-0

src/tests/test_post_office_smtp.py

@@ -15,7 +15,7 @@ class _InMemorySMTPHandler:
     def __init__(self):
         self.envelopes = []
 
-    async def handle_data(self, server, session, envelope):
+    async def handle_DATA(self, server, session, envelope):  # noqa: N802 the name is required by aiosmtpd
         self.envelopes.append(envelope)
         return "250 OK"