Send the Reply-To a message asks for

README.md

@@ -6,6 +6,8 @@ Use it like a standard email backend with an additional feature that allows sett
 
 The `.from_email` attribute must be present in `GETRESPONSE_ADDRESSES` as key, with FromFiledId as value (see settings below).
 
+The same goes for `.reply_to`: GetResponse names a reply-to address by FromFieldId as well, so an address set there has to be registered on the account and listed in `GETRESPONSE_ADDRESSES` too. At most one is supported, because the API holds one.
+
 Result returned from sending mail is an int with extra attribute `getresponse_ids`.
 
 ## Settings

getresponse/mail.py

@@ -88,16 +88,25 @@ def message_to_payload(self, msg):
                 else:
                     raise ValueError("Only single text/html alternative is supported by GetResponse backend.")
 
+        if msg.reply_to:
+            # GetResponse names a reply-to address the way it names a sender, by
+            # the id of a from-field registered on the account, and holds one.
+            if len(msg.reply_to) != 1:
+                raise ValueError("At most one msg.reply_to address is supported by GetResponse backend.")
+            payload['replyTo'] = {
+                'fromFieldId': self.get_sender(msg.reply_to[0]),
+            }
+
         if tag_id := getattr(msg, 'tag_id', None):
             payload['tag'] = {
                 'tagId': tag_id,
             }
         return payload
 
     def get_sender(self, from_email):
-        # if msg.from_email is listed in users settings with FieldId as value, use this address
+        # if the address is listed in users settings with FieldId as value, use this address
         if not settings.GETRESPONSE_ADDRESSES.get(from_email):
-            raise ValueError(f"Given from_email ({from_email}) is not present in GETRESPONSE_ADDRESSES.")
+            raise ValueError(f"Given address ({from_email}) is not present in GETRESPONSE_ADDRESSES.")
         return settings.GETRESPONSE_ADDRESSES.get(from_email)
 
     def attachments_to_payload(self, attachments):

tests/test_payload.py

@@ -0,0 +1,42 @@
+import pytest
+from django.core.mail import EmailMessage
+
+from getresponse.mail import GetResponseBackend
+
+
+@pytest.fixture
+def backend(settings):
+    settings.GETRESPONSE_ADDRESSES = {
+        'webmaster@localhost': 'gr-id-1',
+        'replies@example.com': 'gr-id-2',
+    }
+    return GetResponseBackend()
+
+
+def message(**kwargs):
+    return EmailMessage(
+        subject='Test subject',
+        body='Test body',
+        to=['john.doe@example.com'],
+        **kwargs,
+    )
+
+
+def test_reply_to_travels_as_the_from_field_registered_for_it(backend):
+    payload = backend.message_to_payload(message(reply_to=['replies@example.com']))
+
+    assert payload['replyTo'] == {'fromFieldId': 'gr-id-2'}
+
+
+def test_a_message_without_a_reply_to_sends_none(backend):
+    assert 'replyTo' not in backend.message_to_payload(message())
+
+
+def test_an_unregistered_reply_to_is_refused(backend):
+    with pytest.raises(ValueError):
+        backend.message_to_payload(message(reply_to=['nobody@example.com']))
+
+
+def test_more_than_one_reply_to_is_refused(backend):
+    with pytest.raises(ValueError):
+        backend.message_to_payload(message(reply_to=['replies@example.com', 'webmaster@localhost']))