Validate length of message id when user provided

pyas2lib/as2.py

@@ -357,8 +357,8 @@ def build(
             (default "[email protected]")
 
         :param message_id:
-            The message id to be used for the message. If not provided a
-            unique message id is generated. (default None)
+            A value to be used for the left side of the message id. If not provided a
+              unique id is generated. (default None)
         """
 
         # Validations
@@ -378,13 +378,19 @@ def build(
             )
 
         if message_id:
-            self.message_id = message_id  
+            self.message_id = f"{message_id}@{self.sender.domain}"
         else:
-            # Generate message id using UUID 1 as it uses both hostname and time
             self.message_id = (
                 email_utils.make_msgid(domain=self.sender.domain).lstrip("<").rstrip(">")
             )
 
+        # ensure the total length of the message id is no more than 255 characters
+        if len(self.message_id) > 255:
+            raise ValueError(
+                f"Message ID must be no more than 255 characters for compatibility with some AS2 servers. "
+                f"Current message ID length is {len(self.message_id)}."
+            )
+
         # Set up the message headers
         as2_headers = {
             "AS2-Version": AS2_VERSION,

pyas2lib/tests/test_basic.py

@@ -1,4 +1,5 @@
 """Module for testing the basic features of pyas2."""
+import pytest
 import socket
 from pyas2lib import as2
 from . import Pyas2TestCase
@@ -204,9 +205,21 @@ def test_plain_message_with_custom_message_id(self):
         """Test Message building with a custom message id"""
 
         # Build an As2 message to be transmitted to partner
+        self.org.domain = "example.com"
         out_message = as2.Message(self.org, self.partner)
         out_message.build(self.test_data, message_id="some_custom_id")
-        self.assertEqual(out_message.message_id, "some_custom_id")
+        self.assertEqual(out_message.message_id, "[email protected]")
+
+    def test_invalid_message_id_length_raises_error(self):
+        """Test Message building with a custom message id that's invalid"""
+
+        # Build an As2 message to be transmitted to partner
+        self.org.domain = "example.com"
+        out_message = as2.Message(self.org, self.partner)
+        very_long_message_id = "a" * 1000
+        with pytest.raises(ValueError) as excinfo:
+            out_message.build(self.test_data, message_id=very_long_message_id)
+        assert "Message ID must be no more than 255 characters for compatibility" in str(excinfo.value)
 
     def find_org(self, as2_id):
         return self.org