diff --git a/odoo/addons/base/models/ir_mail_server.py b/odoo/addons/base/models/ir_mail_server.py
index 9c72e54fdbb..ad8f03ca637 100644
--- a/odoo/addons/base/models/ir_mail_server.py
+++ b/odoo/addons/base/models/ir_mail_server.py
@@ -33,6 +33,21 @@ def _print_debug(self, *args):
     _logger.debug(' '.join(str(a) for a in args))
 smtplib.SMTP._print_debug = _print_debug
 
+# Python 3: workaround for bpo-35805, only partially fixed in Python 3.8.
+RFC5322_IDENTIFICATION_HEADERS = {'message-id', 'in-reply-to', 'references', 'resent-msg-id'}
+_noFoldPolicy = email.policy.SMTP.clone(max_line_length=None)
+class IdentificationFieldsNoFoldPolicy(email.policy.EmailPolicy):
+    # Override _fold() to avoid folding identification fields, excluded by RFC2047 section 5
+    # These are particularly important to preserve, as MTAs will often rewrite non-conformant
+    # Message-ID headers, causing a loss of thread information (replies are lost)
+    def _fold(self, name, value, *args, **kwargs):
+        if name.lower() in RFC5322_IDENTIFICATION_HEADERS:
+            return _noFoldPolicy._fold(name, value, *args, **kwargs)
+        return super()._fold(name, value, *args, **kwargs)
+
+# Global monkey-patch for our preferred SMTP policy, preserving the non-default linesep
+email.policy.SMTP = IdentificationFieldsNoFoldPolicy(linesep=email.policy.SMTP.linesep)
+
 # Python 2: replace smtplib's stderr
 class WriteToLogger(object):
     def write(self, s):
diff --git a/odoo/addons/base/tests/test_mail.py b/odoo/addons/base/tests/test_mail.py
index 69ad772447e..9f85bad64f4 100644
--- a/odoo/addons/base/tests/test_mail.py
+++ b/odoo/addons/base/tests/test_mail.py
@@ -450,38 +450,42 @@ class EmailConfigCase(SavepointCase):
 
 class TestEmailMessage(TransactionCase):
     def test_as_string(self):
-        """Ensure all email sent are bpo-34424 free"""
+        """Ensure all email sent are bpo-34424 and bpo-35805 free"""
+
+        message_truth = (
+            r'From: .+? <joe@example\.com>\r\n'
+            r'To: .+? <joe@example\.com>\r\n'
+            r'Message-Id: <[0-9a-z.-]+@[0-9a-z.-]+>\r\n'
+            r'References: (<[0-9a-z.-]+@[0-9a-z.-]+>\s*)+\r\n'
+            r'\r\n'
+        )
 
         class FakeSMTP:
             """SMTP stub"""
             def __init__(this):
                 this.email_sent = False
 
+            # Python 3 before 3.7.4
             def sendmail(this, smtp_from, smtp_to_list, message_str,
                          mail_options=(), rcpt_options=()):
                 this.email_sent = True
-                message_truth = (
-                    r'From: .+? <joe@example\.com>\r\n'
-                    r'To: .+? <joe@example\.com>\r\n'
-                    r'\r\n'
-                )
                 self.assertRegex(message_str, message_truth)
 
+            # Python 3.7.4+
             def send_message(this, message, smtp_from, smtp_to_list,
                              mail_options=(), rcpt_options=()):
                 message_str = message.as_string()
                 this.email_sent = True
-                message_truth = (
-                    r'From: .+? <joe@example\.com>\r\n'
-                    r'To: .+? <joe@example\.com>\r\n'
-                    r'\r\n'
-                )
                 self.assertRegex(message_str, message_truth)
 
         msg = email.message.EmailMessage(policy=email.policy.SMTP)
         msg['From'] = '"Joé Doe" <joe@example.com>'
         msg['To'] = '"Joé Doe" <joe@example.com>'
 
+        # Message-Id & References fields longer than 77 chars (bpo-35805)
+        msg['Message-Id'] = '<929227342217024.1596730490.324691772460938-example-30661-some.reference@test-123.example.com>'
+        msg['References'] = '<345227342212345.1596730777.324691772483620-example-30453-other.reference@test-123.example.com>'
+
         smtp = FakeSMTP()
         self.patch(threading.currentThread(), 'testing', False)
         self.env['ir.mail_server'].send_email(msg, smtp_session=smtp)