Starred emails will remain in your Inbox regardless of the importance of the sender. You can create a Gmail filter that flags emails with a subject of your choice.
1. Go to the filter tab under Gmail's settings page.
2. Click the link to Create a new filter.
3. Enter the text you wish into Subject and click Create filter with this search at the bottom of the window.
4. Click the box by Star it.
5. Click Create Filter.
This will cause our algorithms to leave those in your Inbox until you manually move them out or unstar them.