Be more careful about deleting messages from the MessageListAdapter -

don't do it from something that's not the UI thread

src/com/fsck/k9/activity/MessageList.java

@@ -2287,6 +2287,8 @@ public class MessageList
             }
         }
         public void pruneDirtyMessages()
+        {
+            synchronized (mAdapter.messages)
             {
                 Iterator<MessageInfoHolder> iter = mAdapter.messages.iterator();
                 while (iter.hasNext())
@@ -2299,7 +2301,8 @@ public class MessageList
                             mSelectedCount--;
                             toggleBatchButtons();
                         }
-                    iter.remove();
+                        mAdapter.removeMessage(holder);
+                    }
                 }
                 }
         }