diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index c2cebb1b1f4a5baaafd57b4109e3930c359e9cc3..48c5a55ef41d0057d41da30a300ff57bc67dbc3e 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -1788,7 +1788,7 @@ class MailThread(models.AbstractModel):
                         node.set('src', '/web/image/%s?access_token=%s' % attachment_data)
                         postprocessed = True
                 if postprocessed:
-                    return_values['body'] = lxml.html.tostring(root, pretty_print=False, encoding='UTF-8')
+                    return_values['body'] = lxml.html.tostring(root, pretty_print=False, encoding='unicode')
         return_values['attachment_ids'] = m2m_attachment_ids
         return return_values
 
diff --git a/addons/test_mail/tests/test_mail_gateway.py b/addons/test_mail/tests/test_mail_gateway.py
index 041c211fb9a0e1e010fc4e66ee3503afb6e2524e..84c990a8c446c3c0cf04e2cbb53f158f2cb35efa 100644
--- a/addons/test_mail/tests/test_mail_gateway.py
+++ b/addons/test_mail/tests/test_mail_gateway.py
@@ -8,6 +8,7 @@ from unittest.mock import DEFAULT
 from unittest.mock import patch
 
 from odoo import exceptions
+from odoo.addons.mail.models.mail_thread import MailThread
 from odoo.addons.mail.tests.common import mail_new_test_user
 from odoo.addons.test_mail.data import test_mail_data
 from odoo.addons.test_mail.data.test_mail_data import MAIL_TEMPLATE
@@ -348,7 +349,16 @@ class TestMailgateway(TestMailCommon):
 
     @mute_logger('odoo.addons.mail.models.mail_thread')
     def test_message_process_cid(self):
-        record = self.format_and_process(test_mail_data.MAIL_MULTIPART_IMAGE, self.email_from, 'groups@test.com')
+        origin_message_parse_extract_payload = MailThread._message_parse_extract_payload
+
+        def _message_parse_extract_payload(this, *args, **kwargs):
+            res = origin_message_parse_extract_payload(this, *args, **kwargs)
+            self.assertTrue(isinstance(res['body'], str), 'Body from extracted payload should still be a string.')
+            return res
+
+        with patch.object(MailThread, '_message_parse_extract_payload', _message_parse_extract_payload):
+            record = self.format_and_process(test_mail_data.MAIL_MULTIPART_IMAGE, self.email_from, 'groups@test.com')
+
         message = record.message_ids[0]
         for attachment in message.attachment_ids:
             self.assertIn('/web/image/%s' % attachment.id, message.body)