diff --git a/radicale/acl/IMAP.py b/radicale/acl/IMAP.py
new file mode 100644
index 0000000..e944f4d
--- /dev/null
+++ b/radicale/acl/IMAP.py
@@ -0,0 +1,83 @@
+# -*- coding: utf-8 -*-
+#
+# This file is part of Radicale Server - Calendar Server
+# Copyright © 2012 Daniel Aleksandersen
+# Copyright © 2011 Corentin Le Bail
+# Copyright © 2011 Guillaume Ayoub
+#
+# This library 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 3 of the License, or
+# (at your option) any later version.
+#
+# This library 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 Radicale.  If not, see <http://www.gnu.org/licenses/>.
+
+"""
+IMAP ACL.
+
+Secure authentication based on the ``imaplib`` module.
+
+Validating users against a modern IMAP4rev1 server that awaits STARTTLS on
+port 143. Legacy SSL (often on legacy port 993) is deprecated and thus
+unsupported. STARTTLS is enforced except if host is ``localhost`` as
+passwords are sent in PLAIN.
+
+Python 3.2 or newer is required for TLS.
+
+"""
+
+import imaplib
+
+from radicale import acl, config, log
+
+IMAP_SERVER = config.get("acl", "imap_auth_host_name")
+IMAP_SERVER_PORT = config.get("acl", "imap_auth_host_port")
+
+def has_right(owner, user, password):
+    """Check if ``user``/``password`` couple is valid."""
+
+    if not user or (owner not in acl.PRIVATE_USERS and user != owner):
+        # No user given, or owner is not private and is not user, forbidden
+        return False
+
+    log.LOGGER.debug("[IMAP ACL] Connecting to %s:%s." % (IMAP_SERVER, IMAP_SERVER_PORT,))
+    connection = imaplib.IMAP4(host=IMAP_SERVER, port=IMAP_SERVER_PORT)
+
+    def secure_connection():
+        try:
+            connection.starttls()
+            log.LOGGER.debug("[IMAP ACL] Server connection changed to TLS.")
+            return True
+        except (IMAP4.error, IAMP4.abort) as err:
+            import sys
+            PY_MIN = float(3.2)
+            PY_SYS = float("%s.%s" % (sys.version_info.major, sys.version_info.minor))
+            if PY_SYS < PY_MIN:
+                log.LOGGER.error("[IMAP ACL] Python 3.2 or newer is required for TLS.")
+            log.LOGGER.warning("[IMAP ACL] Server at %s failed to accept TLS connection because of: %s" % (IMAP_SERVER, str(err)))
+        return False # server is not secure
+
+    def server_is_local():
+        if IMAP_HOST == "localhost":
+            log.LOGGER.warning("[IMAP ACL] Server is local. Will allow transmitting unencrypted credentials.")
+            return True
+        return False # server is not local
+
+    """Enforcing security policy of only transmitting credentials over TLS or to local server."""
+    if secure_connection() or server_is_local():
+        try:
+            connection.login( user, password )
+            connection.logout()
+            log.LOGGER.debug("[IMAP ACL] Authenticated user %s via %s." % (user, IMAP_SERVER))
+            return True
+        except ( imaplib.IMAP4.error, imaplib.IMAP4.abort, Exception ) as err:
+            log.LOGGER.error("[IMAP ACL] Server could not authenticate user %s because of: %s" % (user, str(err)))
+    else:
+        log.LOGGER.critical("[IMAP ACL] Server did not support TLS and is not ``localhost``. Refusing to transmit passwords under these conditions. Authentication attempt aborted.")
+    return False # authentication failed
diff --git a/radicale/config.py b/radicale/config.py
index fc545c1..d667771 100644
--- a/radicale/config.py
+++ b/radicale/config.py
@@ -55,6 +55,8 @@ INITIAL_CONFIG = {
         "private_users": "private",
         "htpasswd_filename": "/etc/radicale/users",
         "htpasswd_encryption": "crypt",
+        "imap_auth_host_name": "localhost",
+        "imap_auth_host_port": "143",
         "ldap_url": "ldap://localhost:389/",
         "ldap_base": "ou=users,dc=example,dc=com",
         "ldap_attribute": "uid",