diff --git a/Lib/smtplib.py b/Lib/smtplib.py
index 3295d14..f9d1c99 100755
--- a/Lib/smtplib.py
+++ b/Lib/smtplib.py
@@ -162,7 +162,7 @@
         re.sub(r'(?:\r\n|\n|\r(?!\n))', CRLF, data))
 
 def _quote_periods(bindata):
-    return re.sub(br'(?m)^\.', '..', bindata)
+    return re.sub(br'(?m)^\.', b'..', bindata)
 
 def _fix_eols(data):
     return  re.sub(r'(?:\r\n|\n|\r(?!\n))', CRLF, data)
diff --git a/Lib/test/test_smtplib.py b/Lib/test/test_smtplib.py
index dfe08fa..1204707 100644
--- a/Lib/test/test_smtplib.py
+++ b/Lib/test/test_smtplib.py
@@ -278,6 +278,21 @@
         mexpect = '%s%s\n%s' % (MSG_BEGIN, m.decode('ascii'), MSG_END)
         self.assertEqual(self.output.getvalue(), mexpect)
 
+    def testSendNeedingDotQuote(self):
+        # Issue 12283
+        m = '.A test\n.mes.sage.'
+        smtp = smtplib.SMTP(HOST, self.port, local_hostname='localhost', timeout=3)
+        smtp.sendmail('John', 'Sally', m)
+        # XXX (see comment in testSend)
+        time.sleep(0.01)
+        smtp.quit()
+
+        self.client_evt.set()
+        self.serv_evt.wait()
+        self.output.flush()
+        mexpect = '%s%s\n%s' % (MSG_BEGIN, m, MSG_END)
+        self.assertEqual(self.output.getvalue(), mexpect)
+
     def testSendMessage(self):
         m = email.mime.text.MIMEText('A test message')
         smtp = smtplib.SMTP(HOST, self.port, local_hostname='localhost', timeout=3)
diff --git a/Misc/NEWS b/Misc/NEWS
index 9562646..f422b2d 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -187,6 +187,8 @@
 Library
 -------
 
+- Issue #12283: Fixed regression in smtplib quoting of leading dots in DATA.
+
 - Issue #10424: Argparse now includes the names of the missing required
   arguments in the missing arguments error message.