diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index 2b7b14e..5f54ea1 100644
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -253,8 +253,11 @@
         tmp_file = self._create_tmp()
         try:
             self._dump_message(message, tmp_file)
-        finally:
-            _sync_close(tmp_file)
+        except BaseException:
+            tmp_file.close()
+            os.remove(tmp_file.name)
+            raise
+        _sync_close(tmp_file)
         if isinstance(message, MaildirMessage):
             subdir = message.get_subdir()
             suffix = self.colon + message.get_info()
@@ -700,9 +703,14 @@
     def _append_message(self, message):
         """Append message to mailbox and return (start, stop) offsets."""
         self._file.seek(0, 2)
-        self._pre_message_hook(self._file)
-        offsets = self._install_message(message)
-        self._post_message_hook(self._file)
+        before = self._file.tell()
+        try:
+            self._pre_message_hook(self._file)
+            offsets = self._install_message(message)
+            self._post_message_hook(self._file)
+        except BaseException:
+            self._file.truncate(before)
+            raise
         self._file.flush()
         self._file_length = self._file.tell()  # Record current length of mailbox
         return offsets
@@ -872,7 +880,11 @@
             if self._locked:
                 _lock_file(f)
             try:
-                self._dump_message(message, f)
+                try:
+                    self._dump_message(message, f)
+                except BaseException:
+                    os.remove(new_path)
+                    raise
                 if isinstance(message, MHMessage):
                     self._dump_sequences(message, new_key)
             finally:
diff --git a/Misc/NEWS b/Misc/NEWS
index 15f7f25..4a2ff93 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -37,6 +37,9 @@
 Library
 -------
 
+- Issue #11116: any error during addition of a message to a mailbox now causes
+  a rollback, instead of leaving the mailbox partially modified.
+
 - Issue #8275: Fix passing of callback arguments with ctypes under Win64.
   Patch by Stan Mihai.