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diff --git a/mailman/rules/implicit_dest.py b/mailman/rules/implicit_dest.py
new file mode 100644
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+++ b/mailman/rules/implicit_dest.py
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+# Copyright (C) 2007-2008 by the Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
+# USA.
+
+"""The implicit destination rule."""
+
+__all__ = ['ImplicitDestination']
+__metaclass__ = type
+
+
+import re
+from email.utils import getaddresses
+from zope.interface import implements
+
+from mailman.i18n import _
+from mailman.interfaces import IRule
+
+
+
+class ImplicitDestination:
+    """The implicit destination rule."""
+    implements(IRule)
+
+    name = 'implicit-dest'
+    description = _('Catch messages with implicit destination.')
+    record = True
+
+    def check(self, mlist, msg, msgdata):
+        """See `IRule`."""
+        # Implicit destination checking must be enabled in the mailing list.
+        if not mlist.require_explicit_destination:
+            return False
+        # Messages gated from NNTP will always have an implicit destination so
+        # are never checked.
+        if msgdata.get('fromusenet'):
+            return False
+        # Calculate the list of acceptable aliases.  If the alias starts with
+        # a caret (i.e. ^), then it's a regular expression to match against.
+        aliases = set()
+        alias_patterns = set()
+        for alias in mlist.acceptable_aliases.splitlines():
+            alias = alias.strip().lower()
+            if alias.startswith('^'):
+                alias_patterns.add(alias)
+            elif '@' in alias:
+                aliases.add(alias)
+            else:
+                # This is not a regular expression, nor a fully-qualified
+                # email address, so skip it.
+                pass
+        # Add the list's posting address, i.e. the explicit address, to the
+        # set of acceptable aliases.
+        aliases.add(mlist.posting_address)
+        # Look at all the recipients.  If the recipient is any acceptable
+        # alias (or the explicit posting address), then this rule does not
+        # match.  If not, then add it to the set of recipients we'll check
+        # against the alias patterns later.
+        recipients = set()
+        for header in ('to', 'cc', 'resent-to', 'resent-cc'):
+            for fullname, address in getaddresses(msg.get_all(header, [])):
+                address = address.lower()
+                if address in aliases:
+                    return False
+                recipients.add(address)
+        # Now for all alias patterns, see if any of the recipients matches a
+        # pattern.  If so, then this rule does not match.
+        for pattern in alias_patterns:
+            escaped = re.escape(pattern)
+            for recipient in recipients:
+                try:
+                    if re.match(pattern, recipient, re.IGNORECASE):
+                        return False
+                except re.error:
+                    # The pattern is a malformed regular expression.  Try
+                    # matching again with the pattern escaped.
+                    try:
+                        if re.match(escaped, recipient, re.IGNORECASE):
+                            return False
+                    except re.error:
+                        pass
+        # Nothing matched.
+        return True