From field in the mail headers can be different to the From in the envelope

envelopes/conn.py

@@ -79,15 +79,16 @@ def _connect(self, replace_current=False):
         if self._login:
             self._conn.login(self._login, self._password or '')
 
-    def send(self, envelope):
+    def send(self, envelope, return_path=None):
         """Sends an *envelope*."""
         if not self.is_connected:
             self._connect()
 
         msg = envelope.to_mime_message()
+        smtp_from = return_path or msg["From"]
         to_addrs = [envelope._addrs_to_header([addr]) for addr in envelope._to + envelope._cc + envelope._bcc]
 
-        return self._conn.sendmail(msg['From'], to_addrs, msg.as_string())
+        return self._conn.sendmail(smtp_from, to_addrs, msg.as_string())
 
 
 class GMailSMTP(SMTP):

tests/test_conn.py

@@ -139,3 +139,38 @@ def test_send(self):
         assert call_args[0] == mime_msg['From']
         assert call_args[1] == [envelope._addrs_to_header([addr]) for addr in envelope._to + envelope._cc + envelope._bcc]
         assert call_args[2] != ''
+
+    def test_send(self):
+        conn = SMTP('localhost')
+
+        msg = self._dummy_message()
+        envelope = Envelope(**msg)
+        mime_msg = envelope.to_mime_message()
+
+        conn.send(envelope)
+        assert conn._conn is not None
+        assert len(conn._conn._call_stack.get('sendmail', [])) == 1
+
+        call_args = conn._conn._call_stack['sendmail'][0][0]
+        assert len(call_args) == 3
+        assert call_args[0] == mime_msg['From']
+        assert call_args[1] == [envelope._addrs_to_header([addr]) for addr in envelope._to + envelope._cc + envelope._bcc]
+        assert call_args[2] != ''
+
+    def test_send_with_return_path(self):
+        conn = SMTP('localhost')
+
+        msg = self._dummy_message()
+        envelope = Envelope(**msg)
+        mime_msg = envelope.to_mime_message()
+        FROM_USED_BY_SMTP = "returnpath@domain.tld"
+
+        conn.send(envelope, FROM_USED_BY_SMTP)
+        assert conn._conn is not None
+        assert len(conn._conn._call_stack.get('sendmail', [])) == 1
+
+        call_args = conn._conn._call_stack['sendmail'][0][0]
+        assert len(call_args) == 3
+        assert call_args[0] == FROM_USED_BY_SMTP
+        assert call_args[1] == [envelope._addrs_to_header([addr]) for addr in envelope._to + envelope._cc + envelope._bcc]
+        assert call_args[2] != ''