diff --git a/addons/account/tests/test_account_move_send.py b/addons/account/tests/test_account_move_send.py
index 4361ab0096a..32d489b4140 100644
--- a/addons/account/tests/test_account_move_send.py
+++ b/addons/account/tests/test_account_move_send.py
@@ -466,17 +466,6 @@ class TestAccountMoveSendCommon(AccountTestInvoicingCommon):
 @tagged('post_install_l10n', 'post_install', '-at_install')
 class TestAccountMoveSend(TestAccountMoveSendCommon):
 
-    def test_send_mail_warning_message(self):
-        invoice = self.init_invoice("out_invoice", amounts=[1000], post=True)
-        self.partner_a.email = None
-        wizard = self.create_send_and_print(invoice)
-        self.assertTrue(wizard.send_mail_warning_message)
-
-        # Fix the partner.
-        self.partner_a.email = "turlututu@tsointsoin"
-        wizard.invalidate_recordset(fnames=['send_mail_warning_message'])
-        self.assertFalse(wizard.send_mail_warning_message)
-
     def test_invoice_single(self):
         invoice = self.init_invoice("out_invoice", amounts=[1000], post=True)
         wizard = self.create_send_and_print(invoice)
@@ -491,6 +480,7 @@ class TestAccountMoveSend(TestAccountMoveSendCommon):
             'mail_lang': 'en_US',
             'mail_partner_ids': wizard.move_ids.partner_id.ids,
         }])
+        self.assertFalse(wizard.send_mail_warning_message)
         self.assertTrue(wizard.mail_subject)
         self.assertTrue(wizard.mail_body)
         self._assert_mail_attachments_widget(wizard, [{
@@ -536,6 +526,23 @@ class TestAccountMoveSend(TestAccountMoveSendCommon):
         ])
         self.assertFalse(invoice_attachments)
 
+    def test_invoice_single_readonly_and_checkbox(self):
+        invoice = self.init_invoice("out_invoice", amounts=[1000], post=True)
+
+        self.partner_a.email = None
+        wizard = self.create_send_and_print(invoice)
+        self.assertRecordValues(wizard, [{
+            'send_mail_readonly': True,
+            'checkbox_send_mail': False,
+        }])
+
+        self.partner_a.email = "turlututu@tsointsoin"
+        wizard = self.create_send_and_print(invoice)
+        self.assertRecordValues(wizard, [{
+            'send_mail_readonly': False,
+            'checkbox_send_mail': True,
+        }])
+
     def test_invoice_multi(self):
         invoice1 = self.init_invoice("out_invoice", partner=self.partner_a, amounts=[1000], post=True)
         invoice2 = self.init_invoice("out_invoice", partner=self.partner_b, amounts=[1000], post=True)
@@ -612,6 +619,35 @@ class TestAccountMoveSend(TestAccountMoveSendCommon):
         ])
         self.assertEqual(len(invoice_attachments), 1)
 
+    def test_invoice_multi_readonly_checkbox_warning_message(self):
+        invoice1 = self.init_invoice("out_invoice", partner=self.partner_a, amounts=[1000], post=True)
+        invoice2 = self.init_invoice("out_invoice", partner=self.partner_b, amounts=[1000], post=True)
+
+        self.partner_a.email = None
+        self.partner_b.email = None
+        wizard = self.create_send_and_print(invoice1 + invoice2)
+        self.assertFalse(wizard.send_mail_warning_message)
+        self.assertRecordValues(wizard, [{
+            'send_mail_readonly': True,
+            'checkbox_send_mail': False,
+        }])
+
+        self.partner_a.email = "turlututu@tsointsoin"
+        wizard = self.create_send_and_print(invoice1 + invoice2)
+        self.assertTrue(wizard.send_mail_warning_message)
+        self.assertRecordValues(wizard, [{
+            'send_mail_readonly': False,
+            'checkbox_send_mail': True,
+        }])
+
+        self.partner_b.email = "turlututu@tsointsoin"
+        wizard = self.create_send_and_print(invoice1 + invoice2)
+        self.assertFalse(wizard.send_mail_warning_message)
+        self.assertRecordValues(wizard, [{
+            'send_mail_readonly': False,
+            'checkbox_send_mail': True,
+        }])
+
     def test_invoice_mail_attachments_widget(self):
         invoice = self.init_invoice("out_invoice", amounts=[1000], post=True)
 
diff --git a/addons/account/wizard/account_move_send.py b/addons/account/wizard/account_move_send.py
index 31a976257c3..9da5cad15b3 100644
--- a/addons/account/wizard/account_move_send.py
+++ b/addons/account/wizard/account_move_send.py
@@ -185,29 +185,21 @@ class AccountMoveSend(models.Model):
         for wizard in self:
             wizard.checkbox_download = wizard.enable_download and wizard.company_id.invoice_is_download
 
-    @api.depends('move_ids')
+    @api.depends('mode', 'checkbox_send_mail')
     def _compute_send_mail_extra_fields(self):
         for wizard in self:
             wizard.enable_send_mail = wizard.mode in ('invoice_single', 'invoice_multi')
             wizard.display_mail_composer = wizard.mode == 'invoice_single'
-            send_mail_readonly = False
+            wizard.send_mail_warning_message = False
 
-            display_messages = []
-            if wizard.enable_send_mail:
-                invoices_without_mail_data = wizard.move_ids.filtered(lambda x: not x.partner_id.email)
-                if invoices_without_mail_data:
-                    if wizard.mode == 'invoice_multi':
-                        display_messages.append(_(
-                            "The following invoice(s) will not be sent by email, because the customers don't have email "
-                            "address: "
-                        ))
-                        display_messages.append(", ".join(invoices_without_mail_data.mapped('name')))
-                        send_mail_readonly = True
-                    else:
-                        display_messages.append(_("Please add an email address for your partner"))
+            invoices_without_mail_data = wizard.move_ids.filtered(lambda x: not x.partner_id.email)
+            wizard.send_mail_readonly = invoices_without_mail_data == wizard.move_ids
 
-            wizard.send_mail_readonly = send_mail_readonly
-            wizard.send_mail_warning_message = "".join(display_messages) if display_messages else None
+            if wizard.mode == 'invoice_multi' and wizard.checkbox_send_mail and invoices_without_mail_data:
+                wizard.send_mail_warning_message = _(
+                    "The partners on the following invoices have no email address, "
+                    "so those invoices will not be sent: %s"
+                ) % ", ".join(invoices_without_mail_data.mapped('name'))
 
     @api.depends('move_ids')
     def _compute_checkbox_send_mail(self):
@@ -635,7 +627,7 @@ class AccountMoveSend(models.Model):
                 self._generate_invoice_fallback_documents(errors)
 
             # Send mail.
-            success = {move: move_data for move, move_data in moves_data.items() if not move_data.get('error')}
+            success = {move: move_data for move, move_data in moves_data.items() if not move_data.get('error') and move.partner_id.email}
             if success:
                 self._hook_if_success(success, from_cron=from_cron, allow_fallback_pdf=allow_fallback_pdf)
 
diff --git a/addons/account/wizard/account_move_send_views.xml b/addons/account/wizard/account_move_send_views.xml
index 3d3b1655ac7..ceb89bf38e0 100644
--- a/addons/account/wizard/account_move_send_views.xml
+++ b/addons/account/wizard/account_move_send_views.xml
@@ -17,6 +17,16 @@
                 <field name="send_mail_readonly" invisible="1"/>
                 <field name="display_mail_composer" invisible="1"/>
                 <field name="mail_lang" invisible="1"/>
+                <field name="mail_partner_ids" widget="many2many_tags_email" class="d-none" context="{'force_email': True}"/>
+
+                <!-- Warnings -->
+                <div name="warnings">
+                    <div class="alert alert-warning"
+                        role="alert"
+                        attrs="{'invisible': [('send_mail_warning_message', '=', False)]}">
+                        <field name="send_mail_warning_message"/>
+                    </div>
+                </div>
 
                 <!-- Options -->
                 <div name="options" class="row">
@@ -31,20 +41,16 @@
                             <field name="checkbox_send_mail"
                                 attrs="{'readonly': [('send_mail_readonly', '=', True)]}"/>
                             <b><label for="checkbox_send_mail"/></b>
+                            <i class="fa fa-question-circle ml4"
+                                role="img"
+                                aria-label="Warning"
+                                title="The email address is unknown on the partner"
+                                attrs="{'invisible': [('send_mail_readonly', '=', False)]}"/>
                         </div>
                     </div>
                     <div name="advanced_options" class="col-3"/>
                 </div>
 
-                <!-- Warnings -->
-                <div name="warnings">
-                    <div class="alert alert-warning"
-                        role="alert"
-                        attrs="{'invisible': [('send_mail_warning_message', '=', False)]}">
-                        <field name="send_mail_warning_message"/>
-                    </div>
-                </div>
-
                 <!-- Mail -->
                 <div attrs="{'invisible': [('checkbox_send_mail', '=', False)]}">
                     <group attrs="{'invisible': [('display_mail_composer', '=', False)]}">
diff --git a/addons/snailmail_account/wizard/account_move_send.py b/addons/snailmail_account/wizard/account_move_send.py
index eabec5accf5..feaada9bc45 100644
--- a/addons/snailmail_account/wizard/account_move_send.py
+++ b/addons/snailmail_account/wizard/account_move_send.py
@@ -13,7 +13,9 @@ class AccountMoveSend(models.Model):
         store=True,
         readonly=False,
     )
-    send_by_post_warning_message = fields.Text(compute='_compute_send_by_post_warning_message')
+    send_by_post_cost = fields.Integer(string='Stamps', compute='_compute_send_by_post_extra_fields')
+    send_by_post_warning_message = fields.Text(compute='_compute_send_by_post_extra_fields')
+    send_by_post_readonly = fields.Boolean(compute='_compute_send_by_post_extra_fields')
 
     def _get_available_field_values_in_multi(self, move):
         # EXTENDS 'account'
@@ -31,23 +33,28 @@ class AccountMoveSend(models.Model):
             wizard.enable_send_by_post = wizard.mode in ('invoice_single', 'invoice_multi') \
                 and all(x.state == 'posted' for x in wizard.move_ids)
 
-    @api.depends('send_by_post_warning_message')
+    @api.depends('company_id')
     def _compute_checkbox_send_by_post(self):
         for wizard in self:
-            wizard.checkbox_send_by_post = not wizard.send_by_post_warning_message \
-                and wizard.company_id.invoice_is_snailmail
+            wizard.checkbox_send_by_post = wizard.company_id.invoice_is_snailmail
 
-    @api.depends('mode')
-    def _compute_send_by_post_warning_message(self):
+    @api.depends('mode', 'checkbox_send_by_post')
+    def _compute_send_by_post_extra_fields(self):
         for wizard in self:
-            display_messages = []
-            if wizard.enable_send_by_post:
-                wrong_address_partners = wizard.move_ids.partner_id\
-                    .filtered(lambda x: not self.env['snailmail.letter']._is_valid_address(x))
-                if wrong_address_partners:
-                    display_messages.append(_("The following customers don't have a valid address: "))
-                    display_messages.append(", ".join(wrong_address_partners.mapped('display_name')))
-            wizard.send_by_post_warning_message = "".join(display_messages) if display_messages else None
+            partner_with_valid_address = wizard.move_ids.partner_id \
+                .filtered(self.env['snailmail.letter']._is_valid_address)
+            wizard.send_by_post_cost = len(partner_with_valid_address)
+            wizard.send_by_post_readonly = not partner_with_valid_address
+            wizard.send_by_post_warning_message = False
+
+            if wizard.enable_send_by_post and wizard.checkbox_send_by_post:
+                invoice_without_valid_address = wizard.move_ids.filtered(
+                    lambda move: not self.env['snailmail.letter']._is_valid_address(move.partner_id))
+                if invoice_without_valid_address:
+                    wizard.send_by_post_warning_message = _(
+                        "The partners on the following invoices have no valid address, "
+                        "so those invoices will not be sent: %s"
+                    ) % ", ".join(invoice_without_valid_address.mapped('name'))
 
     # -------------------------------------------------------------------------
     # BUSINESS ACTIONS
diff --git a/addons/snailmail_account/wizard/account_move_send_views.xml b/addons/snailmail_account/wizard/account_move_send_views.xml
index 9306d40a305..35b9d7049af 100644
--- a/addons/snailmail_account/wizard/account_move_send_views.xml
+++ b/addons/snailmail_account/wizard/account_move_send_views.xml
@@ -8,11 +8,24 @@
         <field name="arch" type="xml">
             <xpath expr="//div[@name='standard_options']" position="inside">
                 <field name="enable_send_by_post" invisible="1"/>
+                <field name="send_by_post_readonly" invisible="1"/>
+
                 <div name="option_send_by_post"
                      attrs="{'invisible': [('enable_send_by_post', '=', False)]}">
-                    <field name="checkbox_send_by_post"
-                           attrs="{'readonly': [('send_by_post_warning_message', '!=', False)]}"/>
-                    <b><label for="checkbox_send_by_post"/></b>
+                    <field name="checkbox_send_by_post" attrs="{'readonly': [('send_by_post_readonly', '=', True)]}"/>
+                    <b><label for="checkbox_send_by_post" class="mr4"/></b>
+                    <i class="fa fa-question-circle"
+                        role="img"
+                        aria-label="Warning"
+                        title="The address is unknown on the partner"
+                        attrs="{'invisible': [('send_by_post_readonly', '=', False)]}"/>
+
+                    <span attrs="{'invisible': ['|', ('send_by_post_cost', 'in', (0, 1)), ('checkbox_send_by_post', '=', False)]}">
+                        <b>(
+                            <field name="send_by_post_cost" options="{'digits':[0,0]}" class="mr4"/>
+                            <label for="send_by_post_cost"/>
+                        )</b>
+                    </span>
                 </div>
             </xpath>
             <xpath expr="//div[@name='warnings']" position="inside">