diff --git a/src/mail/app.py b/src/mail/app.py
index 915dd9a..9168876 100644
--- a/src/mail/app.py
+++ b/src/mail/app.py
@@ -543,6 +543,9 @@ class EmailViewerApp(App):
         envelopes_list = self.query_one("#envelopes_list", ListView)
         for i, list_item in enumerate(envelopes_list.children):
             if isinstance(list_item, ListItem):
+                # Bounds check - ListView and message_store may be out of sync during transitions
+                if i >= len(self.message_store.envelopes):
+                    break
                 item_data = self.message_store.envelopes[i]
 
                 if item_data and item_data.get("type") != "header":