VMS Help MAIL Routines, MAIL$MAILFILE_MODIFY, Condition Values Returned  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    MAIL$_ILLFOLNAM    The specified folder name is illegal.
    MAIL$_INVITMCOD    The specified item code is invalid.
    MAIL$_INVITMLEN    The specified item length is invalid.
    MAIL$_MISREQITEM   The required item is missing.
    MAIL$_NOTISAM      The message file is not an indexed file.
    MAIL$_OPENIN       Mail cannot open the file as input.
    SS$_ACCVIO         Access violation.
    Any condition value returned by $CLOSE, $FIND, $PUT, and $UPDATE.