gh-49680: Add translate_line_endings parameter to imaplib append

Pass False to send the message as an exact sequence of octets, without
rewriting bare CR or LF to CRLF.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/imaplib.rst

@@ -195,7 +195,7 @@ upper bound (``'3:*'``).
 An :class:`IMAP4` instance has the following methods:
 
 
-.. method:: IMAP4.append(mailbox, flags, date_time, message)
+.. method:: IMAP4.append(mailbox, flags, date_time, message, *, translate_line_endings=True)
 
    Append *message* to named mailbox.
 
@@ -204,6 +204,14 @@ An :class:`IMAP4` instance has the following methods:
    If *flags* is not already enclosed in parentheses, parentheses are
    added automatically.
 
+   If *translate_line_endings* is true (the default),
+   line endings in *message* are translated to CRLF.
+   Pass ``False`` to send the message literal exactly as given,
+   which is required to preserve messages that contain bare CR or LF.
+
+   .. versionchanged:: next
+      Added the *translate_line_endings* parameter.
+
 
 .. method:: IMAP4.authenticate(mechanism, authobject)
 

Lib/imaplib.py

@@ -483,12 +483,17 @@ def response(self, code):
     #       IMAP4 commands
 
 
-    def append(self, mailbox, flags, date_time, message):
+    def append(self, mailbox, flags, date_time, message, *,
+               translate_line_endings=True):
         """Append message to named mailbox.
 
         (typ, [data]) = <instance>.append(mailbox, flags, date_time, message)
 
                 All args except 'message' can be None.
+
+        If 'translate_line_endings' is true (the default), line endings in
+        'message' are translated to CRLF.  Pass false to send the message
+        literal exactly as given.
         """
         name = 'APPEND'
         if not mailbox:
@@ -502,8 +507,9 @@ def append(self, mailbox, flags, date_time, message):
             date_time = Time2Internaldate(date_time)
         else:
             date_time = None
-        literal = MapCRLF.sub(CRLF, message)
-        self.literal = literal
+        if translate_line_endings:
+            message = MapCRLF.sub(CRLF, message)
+        self.literal = message
         return self._simple_command(name, mailbox, flags, date_time)
 
 

Lib/test/test_imaplib.py

@@ -634,6 +634,25 @@ def test_login(self):
         self.assertEqual(data[0], b'LOGIN completed')
         self.assertEqual(client.state, 'AUTH')
 
+    def test_append_translate_line_endings(self):
+        # By default line endings in the message are normalized to CRLF;
+        # translate_line_endings=False sends the literal exactly (gh-49680).
+        class AppendHandler(SimpleIMAPHandler):
+            def cmd_APPEND(self, tag, args):
+                size = int(args[-1].strip('{}'))
+                self._send_textline('+')
+                self.server.response = self.rfile.read(size)
+                self.rfile.readline()  # trailing CRLF after the literal
+                self._send_tagged(tag, 'OK', 'APPEND completed')
+        message = b'a\rb\nc\r\nd'
+        client, server = self._setup(AppendHandler)
+        client.login('user', 'pass')
+        client.append('INBOX', None, None, message)
+        self.assertEqual(server.response, b'a\r\nb\r\nc\r\nd')
+        client.append('INBOX', None, None, message,
+                      translate_line_endings=False)
+        self.assertEqual(server.response, message)
+
     def test_logout(self):
         client, _ = self._setup(SimpleIMAPHandler)
         typ, data = client.login('user', 'pass')

Misc/NEWS.d/next/Library/2026-07-01-14-00-00.gh-issue-49680.Lm5rQv.rst

@@ -0,0 +1,4 @@
+Add the *translate_line_endings* parameter to :meth:`imaplib.IMAP4.append`.
+By default line endings in the message are translated to CRLF, as before;
+passing ``False`` sends the message literal exactly as given, preserving
+bare CR or LF octets.