Found at a FAQ at the University of Michigan:
How can I mark messages as unread in Pine?
- First you must set the flag command option. From Pine’s main menu:
- Type s (Setup).
- Type c (Config).
- Move the cursor down to highlight the enable-flag-cmd option, and type x to enable it if it is not already.
- Type e to exit the configuration screen.
- Go to your INBOX, highlight the message you want to mark, and type *
- Type n to mark the message as new.