diff --git a/app/exota-milter.py b/app/exota-milter.py
index 9fe7a06..5c0c0f9 100644
--- a/app/exota-milter.py
+++ b/app/exota-milter.py
@@ -9,6 +9,10 @@ import re
 import email.utils
 import authres
 import json
+from policy import (
+  ExOTAPolicyException, ExOTAPolicyNotFoundException, 
+  ExOTAPolicyBackendJSON, ExOTAPolicy
+)
 
 # Globals with mostly senseless defaults ;)
 g_milter_name = 'exota-milter'
@@ -21,7 +25,7 @@ g_milter_dkim_enabled = False
 g_milter_dkim_authservid = 'invalid'
 g_milter_policy_source = 'file' # file, ldap, etc.
 g_milter_policy_file = 'invalid'
-g_milter_policy = {}
+g_milter_policy_backend = None
 
 class ExOTAMilter(Milter.Base):
   # Each new connection is handled in an own thread
@@ -34,6 +38,7 @@ class ExOTAMilter(Milter.Base):
     self.hdr_from = None
     self.hdr_from_domain = None
     self.hdr_tenant_id = None
+    self.hdr_tenant_id_count = 0
     self.dkim_results = []
     self.dkim_valid = False
     # https://stackoverflow.com/a/2257449
@@ -79,6 +84,8 @@ class ExOTAMilter(Milter.Base):
     logging.debug(self.mconn_id + "/" + str(self.queue_id) +
       "/HEADER Header: {0}, Value: {1}".format(name, hval)
     )
+
+    # Parse RFC-5322-From header
     if(name == "From"):
       hdr_5322_from = email.utils.parseaddr(hval)
       self.hdr_from = hdr_5322_from[1].lower()
@@ -93,11 +100,16 @@ class ExOTAMilter(Milter.Base):
       logging.debug(self.mconn_id + "/" + str(self.queue_id) +
         "/HEADER 5322.from: {0}, 5322.from_domain: {1}".format(self.hdr_from, self.hdr_from_domain)
       )
+
+    # Parse non-standardized X-MS-Exchange-CrossTenant-Id header
     elif(name == "X-MS-Exchange-CrossTenant-Id"):
+      self.hdr_tenant_id_count += 1
       self.hdr_tenant_id = hval.lower()
       logging.debug(self.mconn_id + "/" + str(self.queue_id) +
         "/HEADER Tenant-ID: {0}".format(self.hdr_tenant_id)
       )
+
+    # Parse RFC-7601 Authentication-Results header
     elif(name == "Authentication-Results"):
       if g_milter_dkim_enabled == True:
         ar = None
@@ -141,11 +153,19 @@ class ExOTAMilter(Milter.Base):
       )
       self.setreply('550','5.7.1', g_milter_tmpfail_message)
       return Milter.REJECT
-    if self.hdr_from_domain not in g_milter_policy:
-      logging.error(self.mconn_id  + "/" + str(self.queue_id) + "/EOM " +
-        "Could not find 5322.from_domain {0} in policy!".format(self.hdr_from_domain)
+
+    # Get policy for 5322.from_domain
+    policy = None
+    try:
+      policy = g_milter_policy_backend.get(self.hdr_from_domain)
+      logging.debug(self.mconn_id + "/" + self.queue_id +
+        "/EOM Policy for 5322.from_domain={0} fetched from backend".format(self.hdr_from_domain)
       )
-      self.setreply('550','5.7.1', g_milter_reject_message)
+    except (ExOTAPolicyException, ExOTAPolicyNotFoundException) as e:
+      logging.info(self.mconn_id + "/" + self.queue_id +
+        "/EOM {0}".format(e.message)
+      )
+      self.setreply('550','5.7.1', g_milter_tmpfail_message)
       return Milter.REJECT
 
     if self.hdr_tenant_id is None:
@@ -154,18 +174,28 @@ class ExOTAMilter(Milter.Base):
       )
       self.setreply('550','5.7.1', g_milter_reject_message)
       return Milter.REJECT
-    if self.hdr_tenant_id == g_milter_policy[self.hdr_from_domain]['tenant_id'].lower():
+    if self.hdr_tenant_id_count > 1:
       logging.info(self.mconn_id + "/" + self.queue_id +
-        "/EOM: 5322.from_domain={1} tenant_id={0} status=match".format(self.hdr_tenant_id, self.hdr_from_domain)
+        "/EOM: More than one tenant-IDs for {0} found!".format(self.hdr_from_domain)
+      )
+      self.setreply('550','5.7.1', g_milter_reject_message)
+      return Milter.REJECT
+    if self.hdr_tenant_id == policy.get_tenant_id():
+      logging.info(self.mconn_id + "/" + self.queue_id +
+        "/EOM: 5322.from_domain={0} tenant_id={1} status=match".format(
+          self.hdr_from_domain, self.hdr_tenant_id
+        )
       )
     else:
       logging.error(self.mconn_id + "/" + self.queue_id +
-        "/EOM: 5322.from_domain={1} tenant_id={0} status=no_match".format(self.hdr_tenant_id, self.hdr_from_domain)
+        "/EOM: 5322.from_domain={0} tenant_id={1} status=no_match".format(
+          self.hdr_from_domain, self.hdr_tenant_id
+        )
       )
       self.setreply('550','5.7.1', g_milter_reject_message)
       return Milter.REJECT
 
-    if g_milter_dkim_enabled == True and g_milter_policy[self.hdr_from_domain]['dkim'] == True:
+    if g_milter_dkim_enabled and policy.is_dkim_enabled():
       logging.debug(self.mconn_id + "/" + self.queue_id +
         "/EOM: 5322.from_domain={0} dkim_auth=enabled".format(self.hdr_from_domain)
       )
@@ -206,7 +236,7 @@ class ExOTAMilter(Milter.Base):
         
     logging.info(self.mconn_id + "/" + self.queue_id +
       "/EOM: Authentication successful (dkim_enabled={0})".format(
-        str(g_milter_policy[self.hdr_from_domain]['dkim'])
+        str(policy.is_dkim_enabled())
       )
     )
     return Milter.CONTINUE
@@ -260,12 +290,10 @@ if __name__ == "__main__":
     if 'MILTER_POLICY_FILE' in os.environ:
       g_milter_policy_file = os.environ['MILTER_POLICY_FILE']
       try:
-        with open(g_milter_policy_file, 'r') as policy_file:
-          g_milter_policy = json.load(policy_file)
-          policy_file.close()
-          logging.info("Successfully slurped policy file: {0}".format(g_milter_policy_file))
-      except:
-        logging.error("Error reading policy file: " + traceback.format_exc())
+        g_milter_policy_backend = ExOTAPolicyBackendJSON(g_milter_policy_file)
+        logging.info("JSON policy backend initialized")
+      except ExOTAPolicyException as e:
+        logging.error("Policy backend error: {0}".format(e.message))
         sys.exit(1)
     else:
       logging.error("ENV[MILTER_POLICY_FILE] is mandatory!")
diff --git a/app/policy.py b/app/policy.py
new file mode 100644
index 0000000..73b069f
--- /dev/null
+++ b/app/policy.py
@@ -0,0 +1,101 @@
+import json
+import traceback
+import re
+
+class ExOTAPolicyException(Exception):
+  def __init__(self, message):
+    self.message = message
+
+class ExOTAPolicyNotFoundException(ExOTAPolicyException):
+  pass
+
+class ExOTAPolicyInvalidException(ExOTAPolicyException):
+  pass
+
+class ExOTAPolicy():
+  def __init__(self, policy_dict):
+    self.tenant_id = policy_dict['tenant_id']
+    self.dkim = policy_dict['dkim']
+
+  def get_tenant_id(self):
+    return self.tenant_id
+
+  def is_dkim_enabled(self):
+    return self.dkim
+
+  @staticmethod
+  def check_policy(policy_dict):
+    if 'tenant_id' not in policy_dict:
+      raise ExOTAPolicyInvalidException(
+        "Policy must have a 'tenant_id' attribute!"
+      )
+    else:
+      if policy_dict['tenant_id'] == '':
+        raise ExOTAPolicyInvalidException(
+          "'tenant_id' must not be empty!"
+        )
+      if re.match(r'^.*\s+.*$', policy_dict['tenant_id']):
+        raise ExOTAPolicyInvalidException(
+          "'tenant_id' must not contain whitespace characters!"
+        )
+    if 'dkim' not in policy_dict:
+      raise ExOTAPolicyInvalidException(
+        "Policy must have a 'dkim' attribute!"
+      )
+    else:
+      if not isinstance(policy_dict['dkim'], bool):
+        raise ExOTAPolicyInvalidException(
+          "'dkim'({0}) must be boolean!".format(policy_dict['dkim'])
+        )
+
+class ExOTAPolicyBackend():
+  type = None
+  def __init__(self):
+    pass
+  def get(self, from_domain):
+    pass
+
+########## JSON file
+class ExOTAPolicyBackendJSON(ExOTAPolicyBackend):
+  type = 'json'
+  def __init__(self, file_path):
+    self.policies = None
+    try:
+      with open(file_path, 'r') as policy_file:
+        self.policies = json.load(policy_file)
+        policy_file.close()
+        # validate policy
+        for policy in self.policies:
+          try:
+            ExOTAPolicy.check_policy(self.policies[policy])
+          except ExOTAPolicyInvalidException as e:
+            raise ExOTAPolicyException(
+              "Policy {0} is invalid: {1}".format(policy, e.message)
+            ) from e
+    except json.decoder.JSONDecodeError as e:
+      raise ExOTAPolicyException(
+        "JSON-error in policy file: " + str(e)
+      ) from e
+    except Exception as e:
+      raise ExOTAPolicyException(
+        "Error reading policy file: " + traceback.format_exc()
+      ) from e
+  
+  def get(self, from_domain):
+    try:
+      return ExOTAPolicy(self.policies[from_domain])
+    except KeyError as e:
+      raise ExOTAPolicyNotFoundException(
+        "Policy for from_domain={0} not found".format(from_domain)
+      ) from e
+    except Exception as e:
+      raise ExOTAPolicyException(
+        "Error fetching policy for {0}: {1}".format(
+          from_domain, traceback.format_exc()
+        )
+      ) from e
+
+########## LDAP
+class ExOTAPolicyBackendLDAP(ExOTAPolicyBackendJSON):
+  type = 'ldap'
+  pass
\ No newline at end of file
diff --git a/tests/miltertest.lua b/tests/miltertest.lua
index c4a1970..40a17c3 100644
--- a/tests/miltertest.lua
+++ b/tests/miltertest.lua
@@ -33,6 +33,9 @@ end
 if mt.header(conn, "X-MS-Exchange-CrossTenant-Id", "1234abcd-18c5-45e8-88de-123456789abc") ~= nil then
   error "mt.header(Subject) failed"  
 end
+--if mt.header(conn, "X-MS-Exchange-CrossTenant-Id", "4321abcd-18c5-45e8-88de-blahblubb") ~= nil then
+--  error "mt.header(Subject) failed"  
+--end
 if mt.header(conn, "Authentication-Results", "another-wrong-auth-serv-id;\n  dkim=fail header.d=lalalulu.onmicrosoft.com header.s=selector1-lalalulu-onmicrosoft-com header.b=mmmjFpv8") ~= nil then
   error "mt.header(Subject) failed"  
 end
diff --git a/tests/policy.json b/tests/policy.json
index c97e8bb..269d788 100644
--- a/tests/policy.json
+++ b/tests/policy.json
@@ -2,5 +2,9 @@
   "lalalulu.onmicrosoft.com": {
     "tenant_id": "1234abcd-18c5-45e8-88de-123456789abc",
     "dkim": true
+  },
+  "asdf2.onmicrosoft.com": {
+    "tenant_id": "asdftasdfa",
+    "dkim": true
   }
 }
\ No newline at end of file