Library /sys$common/syshlp/mailhelp.hlb MARK, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 message-number

    Indicates the number of the message to be marked or unmarked. The
    message number represents the position of a message in a folder.
    If you specify a number greater than the number of messages in
    the folder, Mail marks the last message in the folder.