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+Acknowledgment headers
+======================
+
+Messages that flow through the global pipeline get their headers 'cooked',
+which basically means that their headers go through several mostly unrelated
+transformations.  Some headers get added, others get changed.  Some of these
+changes depend on mailing list settings and others depend on how the message
+is getting sent through the system.  We'll take things one-by-one.
+
+    >>> from mailman.pipeline.cook_headers import process
+    >>> mlist = config.db.list_manager.create(u'_xtest@example.com')
+    >>> mlist.subject_prefix = u''
+
+When the message's metadata has a 'noack' key set, an 'X-Ack: no' header is
+added.
+
+    >>> msg = message_from_string("""\
+    ... From: aperson@example.com
+    ...
+    ... A message of great import.
+    ... """)
+    >>> process(mlist, msg, dict(noack=True))
+    >>> print msg.as_string()
+    From: aperson@example.com
+    X-Ack: no
+    ...
+
+Any existing X-Ack header in the original message is removed.
+
+    >>> msg = message_from_string("""\
+    ... X-Ack: yes
+    ... From: aperson@example.com
+    ...
+    ... A message of great import.
+    ... """)
+    >>> process(mlist, msg, dict(noack=True))
+    >>> print msg.as_string()
+    From: aperson@example.com
+    X-Ack: no
+    ...