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

       /SUBJECT="subject-text"

    Enables you to specify a different subject when replying to a
    message. Use quotation marks (" ") around the subject heading to
    include more than one word, to print in lowercase letters, or to
    include any spaces or nonalphanumeric characters.