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:

• [N]New
• [D]Deleted
• [*] Important
• [A] Answered

[^T] Can be used to see all the flags and set/unset them one by one (or all).