From Elendor
Syntax: +UNREAD [@<altname>] <messages>
This command marks the indicated messages as new. <messages> can simply be a
message number or it can be any of the syntaxes given in HELP MESSAGES.
Messages which are already marked for deletion may not be marked new.
Marking a message as New simply puts an N next to it in a +SKIM listing
and includes it in the count of unread messages. This can be useful if you
read a message but want to be sure to read it again later.
See Also: +delete, +read