To filter by Subject, you can create a Gmail filter, by clicking here: https://mail.google.com/mail/u/0/?shva=1#settings/filters
1. Click "create a new filter" and enter a unique term (for whatever your actual unique subject term is) into the Subject text field
2. Click "Create filter with this search"
3. Check off "Star it" and click "Create filter"
SaneBox interprets a "Star" to mean "leave it in my Inbox forever" regardless of it's age or importance. If you Star an email in your @SaneLater, we will automatically move that email to your Inbox as well. This is how the "Once" button in your email digest is mechanized.