上文介紹了 Postfwd 的安裝方法。本文就以一個配置例子 (postfwd.cf) 來說明 postfwd 如何應用在 selective greylisting。

# Whitelists
&&TRUSTED_NETS { \
        client_address=192.168.1.0/22 ;   \
        client_address=172.16.128.32/27 ; \
};
&&FREEMAIL { \
        client_name~=\.gmx\.net$ ; \
        client_name~=\.web\.de$ ;  \
        client_name~=\.(aol|yahoo|h(ush|ot)mail)\.com$ ; \
};
&&STATIC { \
        # contains freemailers
        &&FREEMAIL ; \
        client_name~=[\.\-]static[[\.\-] ;               \
        client_name~=^(mail|smtp|mout|mx)[\-]*[0-9]*\. ; \
};
&&DNSWLS { \
        rbl=list.dnswl.org ;                                             \
        rbl=query.bondedsender.org ;                                     \
        rbl=hostkarma.junkemailfilter.com/^127\.0\.0\.1$/3600 ;          \
        rhsbl_client=hostkarma.junkemailfilter.com/^127\.0\.0\.1$/3600 ; \
};

# Spamchecks
&&BADHELO { \
        client_name==!!($$(helo_name)) ; \
};
&&DYNAMIC { \
        client_name==unknown ; \
        client_name~=(\-.+){4} ; \
        client_name~=\d{5} ;     \
        client_name~=[_\.\-]([axt]{0,1}dsl|br(e|oa)dband|ppp|pppoe|dynamic|dynip|ADSL|dial(up|in)|pool|dhcp|leased)[_\.\-] ; \
};
&&DNSBLS { \
        rbl=zen.spamhaus.org ;     \
        rbl=list.dsbl.org ;        \
        rbl=bl.spamcop.net ;       \
        rbl=dnsbl.sorbs.net ;      \
        rbl=ix.dnsbl.manitu.net ;  \
        rhsbl=rddn.dnsbl.net.au ;  \
        rhsbl=rhsbl.ahbl.org ;     \
        rhsbl=rhsbl.sorbs.net ;    \
};


##
## Ruleset
##

# Selective greylisting
id=GREY_001 ;  action=dunno        ;  &&STATIC
id=GREY_002 ;  action=dunno        ;  $$client_name~=$$(sender_domain)$
id=GREY_003 ;  action=dunno        ;  HIT_dnswls>=1
id=GREY_004 ;  action=greylisting  ;  &&DYNAMIC
id=GREY_005 ;  action=greylisting  ;  HIT_dnsbls>=1

# Greylisting should be safe during out-of-office times
id=GREY_006 ;  action=greylisting  ;  days=Sat-Sun
id=GREY_007 ;  action=greylisting  ;  days=Mon-Fri ; time=!!06:00:00-20:00:00

最後一段就是說明怎樣情況下才會對 incoming smtp connection 做 greylisting:

1. 寄件者是用 Dynamic ip 的。 (上述第 28-33 行所定義)

2. 寄件者的 ip address 是在 DNSBL 內。(上述第 34-43 所定義)

3. 辦公時間之外 (星期六日或晚上至凌晨) (上述第 58-59 所定義)