diff --git a/addons/auth_signup/tests/test_reset_password.py b/addons/auth_signup/tests/test_reset_password.py
index e345a647ae7..66168d5d2c8 100644
--- a/addons/auth_signup/tests/test_reset_password.py
+++ b/addons/auth_signup/tests/test_reset_password.py
@@ -18,7 +18,7 @@ class TestResetPassword(HttpCase):
             'email': 'noop@example.com',
         })
 
-        self.assertEqual(test_user.email, url_parse(test_user.signup_url).decode_query()["signup_email"], "query must contain 'signup_email'")
+        self.assertEqual(test_user.email, url_parse(test_user.with_context(create_user=True).signup_url).decode_query()["signup_email"], "query must contain 'signup_email'")
 
         # Invalidate signup_url to skip signup process
         self.env.invalidate_all()
diff --git a/addons/hr_holidays_attendance/models/hr_leave.py b/addons/hr_holidays_attendance/models/hr_leave.py
index 26c67d0aa20..80d43554ddf 100644
--- a/addons/hr_holidays_attendance/models/hr_leave.py
+++ b/addons/hr_holidays_attendance/models/hr_leave.py
@@ -56,7 +56,7 @@ class HRLeave(models.Model):
                 if employee.user_id == self.env.user:
                     raise ValidationError(_('You do not have enough extra hours to request this leave'))
                 raise ValidationError(_('The employee does not have enough extra hours to request this leave.'))
-            if not leave.overtime_id:
+            if not leave.sudo().overtime_id:
                 leave.sudo().overtime_id = self.env['hr.attendance.overtime'].sudo().create({
                     'employee_id': employee.id,
                     'date': fields.Date.today(),
diff --git a/addons/mail/models/mail_mail.py b/addons/mail/models/mail_mail.py
index 5356e525ea4..ffcd806c6bb 100644
--- a/addons/mail/models/mail_mail.py
+++ b/addons/mail/models/mail_mail.py
@@ -84,6 +84,7 @@ class MailMail(models.Model):
     auto_delete = fields.Boolean(
         'Auto Delete',
         help="This option permanently removes any track of email after it's been sent, including from the Technical menu in the Settings, in order to preserve storage space of your Odoo database.")
+    # Unused since v16, to remove in master.
     to_delete = fields.Boolean('To Delete', help='If set, the mail will be deleted during the next Email Queue CRON run.')
     scheduled_date = fields.Datetime('Scheduled Send Date',
         help="If set, the queue manager will send the email after the date. If not set, the email will be send as soon as possible. Unless a timezone is specified, it is considered as being in UTC timezone.")
@@ -111,19 +112,6 @@ class MailMail(models.Model):
             restricted_attaments = mail_sudo.attachment_ids - IrAttachment._filter_attachment_access(mail_sudo.attachment_ids.ids)
             mail_sudo.attachment_ids = restricted_attaments | mail.unrestricted_attachment_ids
 
-    def init(self):
-        """Create a partial index on "to_delete" to make the search on those records fast.
-
-        The benefit on this partial index is to not have a big impact on the
-        update / insert of other records in the database in comparison to a standard
-        index.
-        """
-        self._cr.execute("""
-            CREATE INDEX IF NOT EXISTS mail_mail_to_delete_idx
-                      ON mail_mail(id)
-                   WHERE to_delete = TRUE;
-        """)
-
     @api.model_create_multi
     def create(self, values_list):
         # notification field: if not set, set if mail comes from an existing mail.message
@@ -137,7 +125,7 @@ class MailMail(models.Model):
                 values['scheduled_date'] = False  # void string crashes
         new_mails = super(MailMail, self).create(values_list)
 
-        new_mails_w_attach = self
+        new_mails_w_attach = self.env['mail.mail']
         for mail, values in zip(new_mails, values_list):
             if values.get('attachment_ids'):
                 new_mails_w_attach += mail
@@ -239,8 +227,6 @@ class MailMail(models.Model):
         except Exception:
             _logger.exception("Failed processing mail queue")
 
-        # Remove all the <mail.mail> marked as "to delete"
-        self.env['mail.mail'].sudo().search([('to_delete', '=', True)]).unlink()
         return res
 
     def _postprocess_sent_message(self, success_pids, failure_reason=False, failure_type=None):
@@ -278,7 +264,7 @@ class MailMail(models.Model):
                     # TDE TODO: could be great to notify message-based, not notifications-based, to lessen number of notifs
                     messages._notify_message_notification_update()  # notify user that we have a failure
         if not failure_type or failure_type in ['mail_email_invalid', 'mail_email_missing']:  # if we have another error, we want to keep the mail.
-            self.filtered(lambda mail: mail.auto_delete).to_delete = True
+            self.filtered(lambda mail: mail.auto_delete).unlink()
 
         return True
 
diff --git a/addons/mail/tests/common.py b/addons/mail/tests/common.py
index 4605d50513b..905d09feccf 100644
--- a/addons/mail/tests/common.py
+++ b/addons/mail/tests/common.py
@@ -77,11 +77,6 @@ class MockEmail(common.BaseCase, MockSmtplibCase):
             self.mail_mail_create_mocked = mail_mail_create_mocked
             yield
 
-        if mail_unlink_sent:
-            # Remove all the <mail.mail> marked as to_delete to simulate the CRON
-            # Make tests easier and keep backward compatibility before this patch
-            self.env['mail.mail'].sudo().search([('to_delete', '=', True)]).unlink()
-
     def _init_mail_mock(self):
         self._mails = []
         self._mails_args = []
diff --git a/addons/mail/views/mail_mail_views.xml b/addons/mail/views/mail_mail_views.xml
index af7ae67a357..16270994f6d 100644
--- a/addons/mail/views/mail_mail_views.xml
+++ b/addons/mail/views/mail_mail_views.xml
@@ -46,8 +46,7 @@
                                     <group string="Status">
                                         <field name="auto_delete"
                                             attrs="{'invisible': [('state', '!=', 'outgoing'), ('state', '!=', 'exception')]}"/>
-                                        <field name="to_delete"
-                                            attrs="{'invisible': [('state', '=', 'outgoing')]}"/>
+                                        <field name="to_delete" invisible="1"/>
                                         <field name="is_notification"/>
                                         <field name="message_type"/>
                                         <field name="mail_server_id"/>
@@ -99,7 +98,7 @@
                     <field name="message_type" invisible="1"/>
                     <field name="state" widget="badge" decoration-muted="state in ('sent', 'cancel')"
                         decoration-info="state=='outgoing'" decoration-danger="state=='exception'"/>
-                    <field name="to_delete"/>
+                    <field name="to_delete" invisible="1"/>
                     <button name="send" string="Send Now" type="object" icon="fa-paper-plane" states='outgoing'/>
                     <button name="mark_outgoing" string="Retry" type="object" icon="fa-repeat" states='exception,cancel'/>
                     <button name="cancel" string="Cancel Email" type="object" icon="fa-times-circle" states='outgoing'/>
diff --git a/addons/project/models/project.py b/addons/project/models/project.py
index 4079902b226..904a507a9f0 100644
--- a/addons/project/models/project.py
+++ b/addons/project/models/project.py
@@ -2291,6 +2291,7 @@ class Task(models.Model):
                     record_name=task.display_name,
                     email_layout_xmlid='mail.mail_notification_layout',
                     model_description=task_model_description,
+                    mail_auto_delete=False,
                 )
 
     def _message_auto_subscribe_followers(self, updated_values, default_subtype_ids):
diff --git a/addons/test_mail/tests/test_mail_activity.py b/addons/test_mail/tests/test_mail_activity.py
index 29ac717601f..91d59b93d3c 100644
--- a/addons/test_mail/tests/test_mail_activity.py
+++ b/addons/test_mail/tests/test_mail_activity.py
@@ -589,8 +589,6 @@ class TestActivityMixin(TestActivityCommon):
             origin_2_activity_4 = origin_2_activity_1.copy()
             origin_2_activity_4.date_deadline = datetime(2020, 1, 2, 0, 0, 0)
 
-            self.env['mail.test.activity'].flush_model()
-
             self.assertEqual(origin_2_activity_1.state, 'planned')
             self.assertEqual(origin_2_activity_2.state, 'today')
             self.assertEqual(origin_2_activity_3.state, 'today')
diff --git a/addons/test_mail/tests/test_message_post.py b/addons/test_mail/tests/test_message_post.py
index 9555980e5cb..881d60c2199 100644
--- a/addons/test_mail/tests/test_message_post.py
+++ b/addons/test_mail/tests/test_message_post.py
@@ -1194,7 +1194,6 @@ class TestMessagePostHelpers(TestMessagePostCommon):
                     'message_type': 'email',
                     'model': test_record._name,
                     'notified_partner_ids': self.env['res.partner'],
-                    'to_delete': True,
                     'subtype_id': self.env['mail.message.subtype'],
                     'reply_to': formataddr((f'{self.company_admin.name} {test_record.name}', f'{self.alias_catchall}@{self.alias_domain}')),
                     'res_id': test_record.id,
@@ -1236,7 +1235,6 @@ class TestMessagePostHelpers(TestMessagePostCommon):
                     'model': test_record._name,
                     'notified_partner_ids': self.env['res.partner'],
                     'recipient_ids': test_record.customer_id,
-                    'to_delete': False,
                     'subtype_id': self.env['mail.message.subtype'],
                     'reply_to': formataddr((f'{self.company_admin.name} {test_record.name}', f'{self.alias_catchall}@{self.alias_domain}')),
                     'res_id': test_record.id,
diff --git a/addons/test_mail/tests/test_performance.py b/addons/test_mail/tests/test_performance.py
index ba1b99a8ecd..60a02289a04 100644
--- a/addons/test_mail/tests/test_performance.py
+++ b/addons/test_mail/tests/test_performance.py
@@ -2,6 +2,7 @@
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
 from contextlib import nullcontext
+from unittest.mock import patch
 
 from odoo.addons.base.tests.common import TransactionCaseWithUserDemo
 from odoo.addons.mail.tests.common import MailCommon
@@ -347,7 +348,7 @@ class TestMailAPIPerformance(BaseMailPerformance):
                 'partner_ids': [(4, customer_id)],
             })
 
-        with self.assertQueryCount(admin=29, employee=29):
+        with self.assertQueryCount(admin=35, employee=35):
             composer._action_send_mail()
 
     @users('admin', 'employee')
@@ -368,7 +369,7 @@ class TestMailAPIPerformance(BaseMailPerformance):
                 'partner_ids': [(4, customer.id)],
             })
 
-        with self.assertQueryCount(admin=33, employee=33):
+        with self.assertQueryCount(admin=39, employee=39):
             composer._action_send_mail()
 
     @users('admin', 'employee')
@@ -392,7 +393,7 @@ class TestMailAPIPerformance(BaseMailPerformance):
                 composer_form.attachment_ids.add(attachment)
             composer = composer_form.save()
 
-        with self.assertQueryCount(admin=43, employee=43):  # tm+com 42/42
+        with self.assertQueryCount(admin=49, employee=49):  # tm+com 48/48
             composer._action_send_mail()
 
         # notifications
@@ -436,7 +437,7 @@ class TestMailAPIPerformance(BaseMailPerformance):
                 'partner_ids': [(4, customer_id)],
             })
 
-        with self.assertQueryCount(admin=29, employee=29):
+        with self.assertQueryCount(admin=35, employee=35):
             composer._action_send_mail()
 
     @users('admin', 'employee')
@@ -454,7 +455,7 @@ class TestMailAPIPerformance(BaseMailPerformance):
                 'default_template_id': test_template.id,
             }).create({})
 
-        with self.assertQueryCount(admin=28, employee=28):
+        with self.assertQueryCount(admin=34, employee=34):
             composer._action_send_mail()
 
         # notifications
@@ -478,7 +479,7 @@ class TestMailAPIPerformance(BaseMailPerformance):
                 'default_template_id': test_template.id,
             }).create({})
 
-        with self.assertQueryCount(admin=39, employee=39):
+        with self.assertQueryCount(admin=45, employee=45):
             composer._action_send_mail()
 
         # notifications
@@ -510,7 +511,7 @@ class TestMailAPIPerformance(BaseMailPerformance):
             )
             composer = composer_form.save()
 
-        with self.assertQueryCount(admin=38, employee=38):
+        with self.assertQueryCount(admin=44, employee=44):
             composer._action_send_mail()
 
         # notifications
@@ -540,7 +541,7 @@ class TestMailAPIPerformance(BaseMailPerformance):
             )
             composer = composer_form.save()
 
-        with self.assertQueryCount(admin=59, employee=59):
+        with self.assertQueryCount(admin=65, employee=65):
             composer._action_send_mail()
 
         # notifications
@@ -567,7 +568,7 @@ class TestMailAPIPerformance(BaseMailPerformance):
         # use another user already pre-defined with the email notification type,
         # so the ormcache is preserved.
         record = self.env['mail.test.track'].create({'name': 'Test'})
-        with self.assertQueryCount(admin=25, employee=25):
+        with self.assertQueryCount(admin=37, employee=37):
             record.write({
                 'user_id': self.user_test_email.id,
             })
@@ -650,7 +651,7 @@ class TestMailAPIPerformance(BaseMailPerformance):
     def test_message_post_one_email_notification(self):
         record = self.env['mail.test.simple'].create({'name': 'Test'})
 
-        with self.assertQueryCount(admin=24, employee=24):
+        with self.assertQueryCount(admin=30, employee=30):
             record.message_post(
                 body='<p>Test Post Performances with an email ping</p>',
                 partner_ids=self.customer.ids,
@@ -825,16 +826,23 @@ class TestMailComplexPerformance(BaseMailPerformance):
         } for idx in range(12)])
         mails[-2].write({'email_cc': False, 'email_to': 'strange@example¢¡.com', 'recipient_ids': [(5, 0)]})
         mails[-1].write({'email_cc': False, 'email_to': 'void', 'recipient_ids': [(5, 0)]})
-        with self.assertQueryCount(admin=43, employee=43):
+
+        def _patched_unlink(records):
+            nonlocal unlinked_mails
+            unlinked_mails |= set(records.ids)
+        unlinked_mails = set()
+
+        with (self.assertQueryCount(admin=43, employee=43),
+             patch.object(type(self.env['mail.mail']), 'unlink', _patched_unlink)):
             self.env['mail.mail'].sudo().browse(mails.ids).send()
 
         for mail in mails[:-2]:
             self.assertEqual(mail.state, 'sent')
-            self.assertTrue(mail.to_delete, 'Mail: sent mails are to be unlinked')
+            self.assertIn(mail.id, unlinked_mails, 'Mail: sent mails are to be unlinked')
         self.assertEqual(mails[-2].state, 'exception')
-        self.assertTrue(mails[-2].to_delete, 'Mail: mails with invalid recipient are also to be unlinked')
+        self.assertIn(mails[-2].id, unlinked_mails, 'Mail: mails with invalid recipient are also to be unlinked')
         self.assertEqual(mails[-1].state, 'exception')
-        self.assertTrue(mails[-1].to_delete, 'Mail: mails with invalid recipient are also to be unlinked')
+        self.assertIn(mails[-1].id, unlinked_mails, 'Mail: mails with invalid recipient are also to be unlinked')
 
     @mute_logger('odoo.tests', 'odoo.addons.mail.models.mail_mail', 'odoo.models.unlink')
     @users('admin', 'employee')
@@ -844,7 +852,7 @@ class TestMailComplexPerformance(BaseMailPerformance):
         record = self.container.with_user(self.env.user)
 
         # about 20 (19?) queries per additional customer group
-        with self.assertQueryCount(admin=35, employee=34):
+        with self.assertQueryCount(admin=53, employee=52):
             record.message_post(
                 body='<p>Test Post Performances</p>',
                 message_type='comment',
@@ -862,7 +870,7 @@ class TestMailComplexPerformance(BaseMailPerformance):
         template = self.env.ref('test_mail.mail_test_container_tpl')
 
         # about 20 (19 ?) queries per additional customer group
-        with self.assertQueryCount(admin=42, employee=41):
+        with self.assertQueryCount(admin=60, employee=59):
             record.message_post_with_source(
                 template,
                 message_type='comment',
@@ -952,7 +960,7 @@ class TestMailComplexPerformance(BaseMailPerformance):
         })
         rec1 = rec.with_context(active_test=False)      # to see inactive records
         self.assertEqual(rec1.message_partner_ids, self.partners | self.env.user.partner_id)
-        with self.assertQueryCount(admin=23, employee=23):
+        with self.assertQueryCount(admin=37, employee=37):
             rec.write({'user_id': self.user_portal.id})
         self.assertEqual(rec1.message_partner_ids, self.partners | self.env.user.partner_id | self.user_portal.partner_id)
         # write tracking message
@@ -972,7 +980,7 @@ class TestMailComplexPerformance(BaseMailPerformance):
         customer_id = self.customer.id
         user_id = self.user_portal.id
 
-        with self.assertQueryCount(admin=52, employee=52):
+        with self.assertQueryCount(admin=88, employee=88):
             rec = self.env['mail.test.ticket'].create({
                 'name': 'Test',
                 'container_id': container_id,
@@ -1001,7 +1009,7 @@ class TestMailComplexPerformance(BaseMailPerformance):
         rec1 = rec.with_context(active_test=False)      # to see inactive records
         self.assertEqual(rec1.message_partner_ids, self.user_portal.partner_id | self.env.user.partner_id)
         self.assertEqual(len(rec1.message_ids), 1)
-        with self.assertQueryCount(admin=33, employee=33):
+        with self.assertQueryCount(admin=56, employee=56):
             rec.write({
                 'name': 'Test2',
                 'container_id': self.container.id,
@@ -1038,7 +1046,7 @@ class TestMailComplexPerformance(BaseMailPerformance):
         rec1 = rec.with_context(active_test=False)      # to see inactive records
         self.assertEqual(rec1.message_partner_ids, self.user_portal.partner_id | self.env.user.partner_id)
 
-        with self.assertQueryCount(admin=33, employee=33):
+        with self.assertQueryCount(admin=62, employee=62):
             rec.write({
                 'name': 'Test2',
                 'container_id': container_id,
@@ -1071,7 +1079,7 @@ class TestMailComplexPerformance(BaseMailPerformance):
         rec1 = rec.with_context(active_test=False)      # to see inactive records
         self.assertEqual(rec1.message_partner_ids, self.partners | self.env.user.partner_id | self.user_portal.partner_id)
 
-        with self.assertQueryCount(admin=21, employee=21):
+        with self.assertQueryCount(admin=32, employee=32):
             rec.write({
                 'name': 'Test2',
                 'customer_id': customer_id,
@@ -1301,7 +1309,7 @@ class TestMailHeavyPerformancePost(BaseMailPerformance):
         attachments = self.env['ir.attachment'].with_user(self.env.user).create(self.test_attachments_vals)
         # enable_logging = self.cr._enable_logging() if self.warm else nullcontext()
         # with self.assertQueryCount(employee=49), enable_logging:
-        with self.assertQueryCount(employee=45):
+        with self.assertQueryCount(employee=64):
             record_container.with_context({}).message_post(
                 body='<p>Test body <img src="cid:cid1"> <img src="cid:cid2"></p>',
                 subject='Test Subject',
diff --git a/addons/test_mail_full/tests/test_mail_performance.py b/addons/test_mail_full/tests/test_mail_performance.py
index 478bb733dff..d860a064d73 100644
--- a/addons/test_mail_full/tests/test_mail_performance.py
+++ b/addons/test_mail_full/tests/test_mail_performance.py
@@ -81,7 +81,7 @@ class TestMailPerformance(BaseMailPerformance):
         record_ticket = self.env['mail.test.ticket.mc'].browse(self.record_ticket.ids)
         attachments = self.env['ir.attachment'].create(self.test_attachments_vals)
 
-        with self.assertQueryCount(employee=61):  # tmf: 60
+        with self.assertQueryCount(employee=91):  # tmf: 90
             new_message = record_ticket.message_post(
                 attachment_ids=attachments.ids,
                 body='<p>Test Content</p>',
diff --git a/addons/test_mass_mailing/tests/test_performance.py b/addons/test_mass_mailing/tests/test_performance.py
index 70dec0d4677..c1e9d4942f4 100644
--- a/addons/test_mass_mailing/tests/test_performance.py
+++ b/addons/test_mass_mailing/tests/test_performance.py
@@ -46,17 +46,13 @@ class TestMassMailPerformance(TestMassMailPerformanceBase):
             'mailing_domain': [('id', 'in', self.mm_recs.ids)],
         })
 
-        # runbot needs +2 compared to local
-        with self.assertQueryCount(__system__=427, marketing=428):  # tm 425/426
+        # runbot needs +51 compared to local
+        with self.assertQueryCount(__system__=1523, marketing=1524):
             mailing.action_send_mail()
 
         self.assertEqual(mailing.sent, 50)
         self.assertEqual(mailing.delivered, 50)
 
-        # runbot needs +3 compared to local
-        with self.assertQueryCount(__system__=18, marketing=17):  # tm 15/15
-            self.env['mail.mail'].sudo().search([('to_delete', '=', True)]).unlink()
-
         mails = self.env['mail.mail'].sudo().search([('mailing_id', '=', mailing.id)])
         self.assertFalse(mails, 'Should have auto-deleted the <mail.mail>')
 
@@ -93,16 +89,12 @@ class TestMassMailBlPerformance(TestMassMailPerformanceBase):
             'mailing_domain': [('id', 'in', self.mm_recs.ids)],
         })
 
-        # runbot needs +2 compared to local
-        with self.assertQueryCount(__system__=489, marketing=490):  # tm 487/488
+        # runbot needs +51 compared to local
+        with self.assertQueryCount(__system__=1597, marketing=1598):
             mailing.action_send_mail()
 
         self.assertEqual(mailing.sent, 50)
         self.assertEqual(mailing.delivered, 50)
 
-        # runbot needs +3 compared to local
-        with self.assertQueryCount(__system__=18, marketing=17):  # tm 15/15
-            self.env['mail.mail'].sudo().search([('to_delete', '=', True)]).unlink()
-
         cancelled_mail_count = self.env['mail.mail'].sudo().search([('mailing_id', '=', mailing.id)])
         self.assertEqual(len(cancelled_mail_count), 12, 'Should not have auto deleted the blacklisted emails')