greylisting

http://www.debuntu.org/postfix-and-postgrey-a-proactive-approach-to-spam-filtering

--delay deki sürenin anlamı:
ikinci gönderme bu süreden sonra olursa ikinci göndermeyi kabul et.

posta:~# apt-get install postgrey

posta:~# cat /etc/default/postgrey
# postgrey startup options, created for Debian
# (c)2004 Adrian von Bidder
# Distribute and/or modify at will.

# you may want to set
# --delay=N how long to greylist, seconds (default: 300)
# --max-age=N delete old entries after N days (default: 35)
# see also the postgrey(8) manpage

POSTGREY_OPTS="--inet=127.0.0.1:60000 --delay=50"

# the --greylist-text commandline argument can not be easily passed through
# POSTGREY_OPTS when it contains spaces. So, insert your text here:
#POSTGREY_TEXT="Your customized rejection message here"