diff --git a/mail-filter.py b/mail-filter.py
index 2dbbfd2..15427cf 100644
--- a/mail-filter.py
+++ b/mail-filter.py
@@ -93,15 +93,26 @@ class O365MailFilter(object):
         # within this class as 'self._filter_message()'
         self._filter_message = module.filter_message
 
-    def _load_folders(self):
-        """ retrieve folders for this mailbox and cache their ids """
-        self._folders = {}
-
-        mailbox = self._account.mailbox()
-        folders = mailbox.get_folders()
+    def _load_folders(self, mailbox, folders=None, folder_path=None):
+        """ recursively cache folder IDs for this mailbox """
+        if folders is None:
+            folders = mailbox.get_folders()
+            self._folders = {}
+            folder_path = ''
 
         for folder in folders:
-            self._folders[folder.name] = folder.folder_id
+            if folder_path == '':
+                current_folder_path = f"{folder.name}"
+            else:
+                current_folder_path = f"{folder_path}/{folder.name}"
+
+            if not folder.get_folders():
+                self._folders[current_folder_path] = folder.folder_id
+            else:
+                # add child folders to the cache, because get_folders() doesn't
+                # descend into sub-folders by default
+                self._load_folders(mailbox, folder.get_folders(),
+                                   current_folder_path)
 
     def _clear_cache(self):
         """ clear the filtered message cache """
@@ -120,12 +131,12 @@ class O365MailFilter(object):
         log(f"{self._repr_message(message)} RESULT: {result}")
 
     def filter(self):
-        self._load_folders()
-        self._load_filters()
-
         mailbox = self._account.mailbox()
         inbox = mailbox.inbox_folder()
 
+        self._load_folders(mailbox)
+        self._load_filters()
+
         # set limit to max allowed by O365, which is 999 messages
         # we have to explicitly set a limit value when calling get_messages() or
         # the O365 library will not paginate results correctly