diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 73c3e6f8c8b..a26fc7fbd6f 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -2236,7 +2236,7 @@ class MailThread(models.AbstractModel):
                         node.set('src', f'/web/image/{att_id}?access_token={token}')
                         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 e3206f9e78d..f4989fe8da8 100644
--- a/addons/test_mail/tests/test_mail_gateway.py
+++ b/addons/test_mail/tests/test_mail_gateway.py
@@ -11,6 +11,7 @@ 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, MailCommon
 from odoo.addons.test_mail.data import test_mail_data
 from odoo.addons.test_mail.data.test_mail_data import MAIL_TEMPLATE
@@ -216,7 +217,15 @@ class TestMailgateway(MailCommon):
 
     @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, f'groups@{self.alias_domain}')
+        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, f'groups@{self.alias_domain}')
         message = record.message_ids[0]
         for attachment in message.attachment_ids:
             self.assertIn('/web/image/%s' % attachment.id, message.body)