diff --git a/odoo/addons/base/tests/common.py b/odoo/addons/base/tests/common.py
index 67608368a7a..3f85b96944a 100644
--- a/odoo/addons/base/tests/common.py
+++ b/odoo/addons/base/tests/common.py
@@ -322,6 +322,9 @@ class MockSmtplibCase:
             def login(self, user, password):
                 pass
 
+            def starttls(self, keyfile=None, certfile=None, context=None):
+                pass
+
         self.testing_smtp_session = TestingSMTPSession()
 
         IrMailServer = self.env['ir.mail_server']
diff --git a/odoo/addons/base/tests/test_ir_mail_server.py b/odoo/addons/base/tests/test_ir_mail_server.py
index c2a330067f9..46d832fa14a 100644
--- a/odoo/addons/base/tests/test_ir_mail_server.py
+++ b/odoo/addons/base/tests/test_ir_mail_server.py
@@ -254,6 +254,20 @@ class TestIrMailServer(TransactionCase, MockSmtplibCase):
             from_filter='custom_domain.com',
         )
 
+        # Test when forcing the mail server and when smtp_encryption is "starttls"
+        self.server_domain.smtp_encryption = "starttls"
+        with self.mock_smtplib_connection():
+            message = self._build_email(mail_from='specific_user@test.com')
+            IrMailServer.send_email(message, mail_server_id=self.server_domain.id)
+
+        self.connect_mocked.assert_called_once()
+        self.assert_email_sent_smtp(
+            smtp_from='specific_user@test.com',
+            message_from='specific_user@test.com',
+            from_filter='test.com',
+        )
+
+
     @mute_logger('odoo.models.unlink')
     def test_mail_server_send_email_smtp_session(self):
         """Test all the cases when we provide the SMTP session.