diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index de9da391248..27dd0db3c1e 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -146,6 +146,19 @@ class Generator:
         # BytesGenerator overrides this to encode strings to bytes.
         return s
 
+    def _write_lines(self, lines):
+        # We have to transform the line endings.
+        if not lines:
+            return
+        lines = lines.splitlines(True)
+        for line in lines[:-1]:
+            self.write(line.rstrip('\r\n'))
+            self.write(self._NL)
+        laststripped = lines[-1].rstrip('\r\n')
+        self.write(laststripped)
+        if len(lines[-1])!=len(laststripped):
+            self.write(self._NL)
+
     def _write(self, msg):
         # We can't write the headers yet because of the following scenario:
         # say a multipart message includes the boundary string somewhere in
@@ -217,7 +230,7 @@ class Generator:
                 payload = msg.get_payload()
         if self._mangle_from_:
             payload = fcre.sub('>From ', payload)
-        self.write(payload)
+        self._write_lines(payload)
 
     # Default body handler
     _writeBody = _handle_text
@@ -256,7 +269,8 @@ class Generator:
                 preamble = fcre.sub('>From ', msg.preamble)
             else:
                 preamble = msg.preamble
-            self.write(preamble + self._NL)
+            self._write_lines(preamble)
+            self.write(self._NL)
         # dash-boundary transport-padding CRLF
         self.write('--' + boundary + self._NL)
         # body-part
@@ -278,7 +292,7 @@ class Generator:
                 epilogue = fcre.sub('>From ', msg.epilogue)
             else:
                 epilogue = msg.epilogue
-            self.write(epilogue)
+            self._write_lines(epilogue)
 
     def _handle_multipart_signed(self, msg):
         # The contents of signed parts has to stay unmodified in order to keep
@@ -402,7 +416,7 @@ class BytesGenerator(Generator):
         if _has_surrogates(msg._payload) and not self.policy.cte_type=='7bit':
             if self._mangle_from_:
                 msg._payload = fcre.sub(">From ", msg._payload)
-            self.write(msg._payload)
+            self._write_lines(msg._payload)
         else:
             super(BytesGenerator,self)._handle_text(msg)
 
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 593d27b5c2d..dd54a1cd32a 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -3111,6 +3111,40 @@ multipart/report
             email.utils.make_msgid(domain='testdomain-string')[-19:],
             '@testdomain-string>')
 
+    def test_Generator_linend(self):
+        # Issue 14645.
+        with openfile('msg_26.txt', newline='\n') as f:
+            msgtxt = f.read()
+        msgtxt_nl = msgtxt.replace('\r\n', '\n')
+        msg = email.message_from_string(msgtxt)
+        s = StringIO()
+        g = email.generator.Generator(s)
+        g.flatten(msg)
+        self.assertEqual(s.getvalue(), msgtxt_nl)
+
+    def test_BytesGenerator_linend(self):
+        # Issue 14645.
+        with openfile('msg_26.txt', newline='\n') as f:
+            msgtxt = f.read()
+        msgtxt_nl = msgtxt.replace('\r\n', '\n')
+        msg = email.message_from_string(msgtxt_nl)
+        s = BytesIO()
+        g = email.generator.BytesGenerator(s)
+        g.flatten(msg, linesep='\r\n')
+        self.assertEqual(s.getvalue().decode('ascii'), msgtxt)
+
+    def test_BytesGenerator_linend_with_non_ascii(self):
+        # Issue 14645.
+        with openfile('msg_26.txt', 'rb') as f:
+            msgtxt = f.read()
+        msgtxt = msgtxt.replace(b'with attachment', b'fo\xf6')
+        msgtxt_nl = msgtxt.replace(b'\r\n', b'\n')
+        msg = email.message_from_bytes(msgtxt_nl)
+        s = BytesIO()
+        g = email.generator.BytesGenerator(s)
+        g.flatten(msg, linesep='\r\n')
+        self.assertEqual(s.getvalue(), msgtxt)
+
 
 # Test the iterator/generators
 class TestIterators(TestEmailBase):
diff --git a/Misc/NEWS b/Misc/NEWS
index 4bdee9da0ec..f15165147eb 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -275,6 +275,11 @@ Core and Builtins
 Library
 -------
 
+- Issue #14645: The email generator classes now produce output using the
+  specified linesep throughout.  Previously if the prolog, epilog, or
+  body were stored with a different linesep, that linesep was used.  This
+  fix corrects an RFC non-compliance issue with smtplib.send_message.
+
 - Issue #17278: Fix a crash in heapq.heappush() and heapq.heappop() when
   the list is being resized concurrently.