diff --git a/addons/account/models/ir_attachment.py b/addons/account/models/ir_attachment.py
index ddaf4b40ac7..516f955675d 100644
--- a/addons/account/models/ir_attachment.py
+++ b/addons/account/models/ir_attachment.py
@@ -97,9 +97,10 @@ class IrAttachment(models.Model):
         """
 
         def is_xml(attachment):
-            # XML attachments received by mail have a 'text/plain' mimetype.
-            # Therefore, if content start with '<?xml', it is considered as XML.
-            is_text_plain_xml = 'text/plain' in attachment.mimetype and attachment.raw.startswith(b'<?xml')
+            # XML attachments received by mail have a 'text/plain' mimetype (cfr. context key:
+            # 'attachments_mime_plainxml'). Therefore, if content start with '<?xml', or if the filename ends with
+            # '.xml', it is considered as XML.
+            is_text_plain_xml = 'text/plain' in attachment.mimetype and (attachment.raw.startswith(b'<?xml') or attachment.name.endswith('.xml'))
             return attachment.mimetype.endswith('/xml') or is_text_plain_xml
 
         return [
diff --git a/addons/account_edi_ubl_cii/models/account_edi_common.py b/addons/account_edi_ubl_cii/models/account_edi_common.py
index b9d3ac9b9fa..5bf7d56d41e 100644
--- a/addons/account_edi_ubl_cii/models/account_edi_common.py
+++ b/addons/account_edi_ubl_cii/models/account_edi_common.py
@@ -281,7 +281,14 @@ class AccountEdiCommon(models.AbstractModel):
         else:
             return
         if not new and invoice.move_type != move_type:
-            return
+            # with an email alias to create account_move, first the move is created (using alias_defaults, which
+            # contains move_type = 'out_invoice') then the attachment is decoded, if it represents a credit note,
+            # the move type needs to be changed to 'out_refund'
+            types = {move_type, invoice.move_type}
+            if types == {'out_invoice', 'out_refund'} or types == {'in_invoice', 'in_refund'}:
+                invoice.move_type = move_type
+            else:
+                return
 
         # Update the invoice.
         invoice.move_type = move_type
@@ -317,7 +324,7 @@ class AccountEdiCommon(models.AbstractModel):
                 # (Windows or Linux style) and/or the name of the xml instead of the pdf.
                 # Get only the filename with a pdf extension.
                 name = attachment_name.text.split('\\')[-1].split('/')[-1].split('.')[0] + '.pdf'
-                attachments |= self.env['ir.attachment'].create({
+                attachment = self.env['ir.attachment'].create({
                     'name': name,
                     'res_id': invoice.id,
                     'res_model': 'account.move',
@@ -325,6 +332,13 @@ class AccountEdiCommon(models.AbstractModel):
                     'type': 'binary',
                     'mimetype': 'application/pdf',
                 })
+                # Upon receiving an email (containing an xml) with a configured alias to create invoice, the xml is
+                # set as the main_attachment. To be rendered in the form view, the pdf should be the main_attachment.
+                if invoice.message_main_attachment_id and \
+                        invoice.message_main_attachment_id.name.endswith('.xml') and \
+                        'pdf' not in invoice.message_main_attachment_id.mimetype:
+                    invoice.message_main_attachment_id = attachment
+                attachments |= attachment
         if attachments:
             invoice.with_context(no_new_invoice=True).message_post(attachment_ids=attachments.ids)
 
diff --git a/addons/l10n_account_edi_ubl_cii_tests/tests/test_xml_ubl_be.py b/addons/l10n_account_edi_ubl_cii_tests/tests/test_xml_ubl_be.py
index 3aa4377b3a5..3f7cff8ad73 100644
--- a/addons/l10n_account_edi_ubl_cii_tests/tests/test_xml_ubl_be.py
+++ b/addons/l10n_account_edi_ubl_cii_tests/tests/test_xml_ubl_be.py
@@ -369,3 +369,18 @@ class TestUBLBE(TestUBLCommon):
         # source: vat-category-E.xml
         self._assert_imported_invoice_from_file(subfolder=subfolder, filename='bis3_tax_exempt_gbp.xml',
             amount_total=1200, amount_tax=0, list_line_subtotals=[1200], currency_id=self.env.ref('base.GBP').id)
+
+    def test_import_existing_invoice_flip_move_type(self):
+        """ Tests whether the move_type of an existing invoice can be flipped when importing an attachment
+        For instance: with an email alias to create account_move, first the move is created (using alias_defaults,
+        which contains move_type = 'out_invoice') then the attachment is decoded, if it represents a credit note,
+        the move type needs to be changed to 'out_refund'
+        """
+        invoice = self.env['account.move'].create({'move_type': 'out_invoice'})
+        self._update_invoice_from_file(
+            'l10n_account_edi_ubl_cii_tests',
+            'tests/test_files/from_odoo',
+            'bis3_out_refund.xml',
+            invoice,
+        )
+        self.assertRecordValues(invoice, [{'move_type': 'out_refund', 'amount_total': 3164.22}])
diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index cb40936dae9..1f08f6504a7 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -2188,7 +2188,10 @@ class MailThread(models.AbstractModel):
 
                 if isinstance(content, str):
                     encoding = info and info.get('encoding')
-                    content = content.encode(encoding or 'utf-8')
+                    try:
+                        content = content.encode(encoding or "utf-8")
+                    except UnicodeEncodeError:
+                        content = content.encode("utf-8")
                 elif isinstance(content, EmailMessage):
                     content = content.as_bytes()
                 elif content is None:
diff --git a/addons/test_mail/data/test_mail_data.py b/addons/test_mail/data/test_mail_data.py
index f017f24bc5c..9baf09c3c5b 100644
--- a/addons/test_mail/data/test_mail_data.py
+++ b/addons/test_mail/data/test_mail_data.py
@@ -276,6 +276,56 @@ SGVsbG8gd29ybGQK
 --Apple-Mail=_9331E12B-8BD2-4EC7-B53E-01F3FBEC9227--
 """
 
+MAIL_MULTIPART_INVALID_ENCODING = """Return-Path: <whatever-2a840@postmaster.twitter.com>
+To: {to}
+cc: {cc}
+Received: by mail1.openerp.com (Postfix, from userid 10002)
+    id 5DF9ABFB2A; Fri, 10 Aug 2012 16:16:39 +0200 (CEST)
+From: {email_from}
+Subject: {subject}
+MIME-Version: 1.0
+Content-Type: multipart/alternative;
+    boundary="00000000000005d9da05fa394cc0"
+Date: Fri, 10 Aug 2012 14:16:26 +0000
+Message-ID: {msg_id}
+{extra}
+
+--00000000000005d9da05fa394cc0
+Content-Type: multipart/alternative; boundary="00000000000005d9d905fa394cbe"
+
+--00000000000005d9d905fa394cbe
+Content-Type: text/plain; charset="UTF-8"
+
+Dear customer,
+
+Please find attached the Peppol Bis 3 attachment of your invoice (with an
+encoding error in the address)
+
+Cheers,
+
+--00000000000005d9d905fa394cbe
+Content-Type: text/html; charset="UTF-8"
+
+<div dir="ltr">Dear customer,<div><br></div><div>Please find attached the Peppol Bis 3 attachment of your invoice (with an encoding error in the address)</div><div><br></div><div>Cheers,</div></div>
+
+--00000000000005d9d905fa394cbe--
+
+--00000000000005d9da05fa394cc0
+Content-Type: text/xml; charset="US-ASCII";
+ name="bis3_with_error_encoding_address.xml"
+Content-Disposition: attachment; 
+	filename="bis3_with_error_encoding_address.xml"
+Content-Transfer-Encoding: base64
+Content-ID: <f_lgxgdqx40>
+X-Attachment-Id: f_lgxgdqx40
+
+PEludm9pY2UgeG1sbnM6Y2JjPSJ1cm46b2FzaXM6bmFtZXM6c3BlY2lmaWNhdGlvbjp1Ymw6c2No
+ZW1hOnhzZDpDb21tb25CYXNpY0NvbXBvbmVudHMtMiIgeG1sbnM9InVybjpvYXNpczpuYW1lczpz
+cGVjaWZpY2F0aW9uOnVibDpzY2hlbWE6eHNkOkludm9pY2UtMiI+DQo8Y2JjOlN0cmVldE5hbWU+
+Q2hhdXNz77+977+9ZSBkZSBCcnV4ZWxsZXM8L2NiYzpTdHJlZXROYW1lPg0KPC9JbnZvaWNlPg0K
+--00000000000005d9da05fa394cc0--
+"""
+
 
 MAIL_SINGLE_BINARY = """X-Original-To: raoul@grosbedon.fr
 Delivered-To: raoul@grosbedon.fr
diff --git a/addons/test_mail/tests/test_mail_gateway.py b/addons/test_mail/tests/test_mail_gateway.py
index bb2d4db482a..534dccef77c 100644
--- a/addons/test_mail/tests/test_mail_gateway.py
+++ b/addons/test_mail/tests/test_mail_gateway.py
@@ -1742,6 +1742,27 @@ class TestMailgateway(MailCommon):
         records = self.env['mail.test.gateway'].search([('name', 'ilike', 'Whitelist test alias loop %')])
         self.assertEqual(len(records), 10, msg='Email whitelisted should not have the restriction')
 
+    # --------------------------------------------------
+    # Corner cases / Bugs during message process
+    # --------------------------------------------------
+
+    def test_message_process_file_encoding_ascii(self):
+        """ Incoming email containing an xml attachment with unknown characters (�) but an ASCII charset should not
+        raise an Exception. UTF-8 is used as a safe fallback.
+        """
+        record = self.format_and_process(test_mail_data.MAIL_MULTIPART_INVALID_ENCODING, self.email_from, 'groups@test.com')
+
+        self.assertEqual(record.message_ids.attachment_ids.name, 'bis3_with_error_encoding_address.xml')
+        # NB: the xml received by email contains b"Chauss\xef\xbf\xbd\xef\xbf\xbde" with "\xef\xbf\xbd" being the
+        # replacement character � in UTF-8.
+        # When calling `_message_parse_extract_payload`, `part.get_content()` will be called on the attachment part of
+        # the email, triggering the decoding of the base64 attachment, so b"Chauss\xef\xbf\xbd\xef\xbf\xbde" is
+        # first retrieved. Then, `get_text_content` in `email` tries to decode this using the charset of the email
+        # part, i.e: `content.decode('us-ascii', errors='replace')`. So the errors are replaced using the Unicode
+        # replacement marker and the string "Chauss������e" is used to create the attachment.
+        # This explains the multiple "�" in the attachment.
+        self.assertIn("Chauss������e de Bruxelles", record.message_ids.attachment_ids.raw.decode())
+
 
 @tagged('mail_gateway')
 class TestMailThreadCC(MailCommon):