To mark a message as unread in pine this can be done using flags.
First flags have to be turned on in setup
[M]ain [S]etup [C]onfig
Then find “enable-flag-cmd” and turn it on.
After this marking a message as unread is done by “* N” in the list view or when looking at the message.
All the flags are:
- [*] Important
- [A] Answered
[^T] Can be used to see all the flags and set/unset them one by one (or all).