This function causes a store to delete the specified
flag to the flags set for the
messages in the specified sequence.
The flags which you can unset are "\\Seen", "\\Answered",
"\\Flagged", "\\Deleted", and "\\Draft" (as defined
by RFC2060). Возвращает TRUE в случае успешного завершения или FALSE в случае возникновения ошибки..
options are a bit mask and may contain
the single option:
ST_UID - The sequence argument contains UIDs
instead of sequence numbers