diff -ur mailman-2.1.20.org/Mailman/Gui/General.py mailman-2.1.20/Mailman/Gui/General.py
--- mailman-2.1.20.org/Mailman/Gui/General.py	2015-03-31 19:21:49.000000000 +0200
+++ mailman-2.1.20/Mailman/Gui/General.py	2016-02-21 10:25:57.685943034 +0100
@@ -505,6 +505,13 @@
                  here."""))
                 )
 
+        # add X-IMAP-Folder header
+        rtn.append(
+            ('x_imap_folder', mm_cfg.String, WIDTH, 0,
+             _('''If filled add to each outgroing email X-IMAP-Folder header
+             with this value.'''))
+            )
+
         # Discard held messages after this number of days
         rtn.append(
             ('max_days_to_hold', mm_cfg.Number, 7, 0,
diff -ur mailman-2.1.20.org/Mailman/Handlers/CookHeaders.py mailman-2.1.20/Mailman/Handlers/CookHeaders.py
--- mailman-2.1.20.org/Mailman/Handlers/CookHeaders.py	2015-03-31 19:21:49.000000000 +0200
+++ mailman-2.1.20/Mailman/Handlers/CookHeaders.py	2016-02-21 10:25:57.685943034 +0100
@@ -107,6 +107,11 @@
     # X-BeenThere's intact.
     change_header('X-BeenThere', mlist.GetListEmail(),
                   mlist, msg, msgdata, delete=False)
+
+    # X-IMAP-Folder header, used commonly for server side filtering to folders.
+    if mlist.x_imap_folder:
+        msg['X-IMAP-Folder'] = mlist.x_imap_folder
+
     # Add Precedence: and other useful headers.  None of these are standard
     # and finding information on some of them are fairly difficult.  Some are
     # just common practice, and we'll add more here as they become necessary.
diff -ur mailman-2.1.20.org/Mailman/MailList.py mailman-2.1.20/Mailman/MailList.py
--- mailman-2.1.20.org/Mailman/MailList.py	2015-03-31 19:21:49.000000000 +0200
+++ mailman-2.1.20/Mailman/MailList.py	2016-02-21 10:25:57.685943034 +0100
@@ -367,6 +367,7 @@
         self.available_languages = []
         self.include_rfc2369_headers = 1
         self.include_list_post_header = 1
+        self.x_imap_folder = ''
         self.include_sender_header = 1
         self.filter_mime_types = mm_cfg.DEFAULT_FILTER_MIME_TYPES
         self.pass_mime_types = mm_cfg.DEFAULT_PASS_MIME_TYPES
diff -ur mailman-2.1.20.org/Mailman/versions.py mailman-2.1.20/Mailman/versions.py
--- mailman-2.1.20.org/Mailman/versions.py	2015-03-31 19:21:49.000000000 +0200
+++ mailman-2.1.20/Mailman/versions.py	2016-02-21 10:26:55.443940355 +0100
@@ -168,6 +168,10 @@
         else:
             return None
 
+    def add_only_if_missing(attr, initval, l=l):
+        if not hasattr(l, attr):
+            setattr(l, attr, initval)
+
     # Migrate to 2.1b3, baw 17-Aug-2001
     if hasattr(l, 'dont_respond_to_post_requests'):
         oldval = getattr(l, 'dont_respond_to_post_requests')
@@ -414,6 +418,8 @@
     PreferStored('author_is_list', 'from_is_list',
                  mm_cfg.DEFAULT_FROM_IS_LIST)
 
+    add_only_if_missing('x_imap_folder', '')
+
 
 
 def NewVars(l):