This is the logic behind blocking "subscriber" networks by PTR hostname and by HELO hostname. Your MX is receiving 10's of 1000's of spams from charter/att/comcast/rr/telus/pacbell/sbd/etc/etc/etc networks, for every handful, if any, of valid msgs, so you block all those subscriber networks until those multi-$$billion network factories, which huge, highly competent, highly paid IT staffs, stop polluting your email environment.
From another list, someone toted up his volumes of spam, note the dominance of subscriber networks:
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The perennial question here seems to be "where does it all come from", and in the spirit of answering that question, here are the top however many sending domains/cc.tlds for messages we rejected as spam (which, given how much we reject, is a lot more accurate than "spam we received") since 1/1/04 (domains/cc.tlds with more than 5 attempts):
585 comcast.net
402 rr.com
188 attbi.com
175 pacbell.net
165 ameritech.net
130 shawcable.net
128 adelphia.net
125 optonline.net
106 wanadoo.fr
105 verizon.net
103 bellsouth.net
89 charter.com
88 dsl-verizon.net
80 t-dialin.net
79 swbell.net
63 ne.jp
61 videotron.ca
58 net.il
51 proxad.net
48 com.tw
48 a2000.nl
46 tiscali.nl
46 co.uk
43 yahoo.com
41 rogers.com
40 mchsi.com
38 cgocable.net
36 snet.net
35 mindspring.com
34 interbusiness.it
32 surfer.at
30 telus.net
30 go2lnk.com
30 com.br
29 net.au
28 rima-tde.net
27 wideopenwest.com
24 bbtt.de
22 nuvox.net
21 com.hk
21 bbtec.net
20 telia.com
20 charter-stl.com
19 hispeed.ch
19 dsl.net
19 btopenworld.com
18 covad.net
18 btcentralplus.com
17 uu.net
17 upenn.edu
17 proper.com
17 net.ua
16 wanadoo.nl
15 xo.net
15 tiscali.com
15 telepacific.net
15 menta.net
15 frontiernet.net
15 cox-internet.com
14 tpnet.pl
14 skynet.be
14 qwest.net
14 nf.net
14 net.mx
14 isdn.net
14 insightBB.com
14 com.au
13 waltadams.com
13 cnc.net
13 bezeqint.net
12 transedge.com
12 t-ipconnect.de
12 freeserve.com
12 club-internet.fr
12 chello.nl
12 btitelecom.net
11 skanova.com
11 ntl.com
11 noos.fr
11 krneki.org
11 ip.pt
11 infocrossing.net
11 chartertn.net
10 theplanet.com
10 tele.dk
10 slovanet.sk
10 pajo.com
10 one-2.net
10 euskaltel.es
10 btinternet.com
9 vasoffers.com
9 telecom.sk
9 sprint-hsd.net
9 pipex.com
9 odessa.edu
9 mun.ca
9 listbox.com
9 com.ar
8 twtelecom.net
8 quolia.com
8 nexicom.net
8 net.co
8 net.br
8 conxion.com
8 acedsl.com
7 subscribermails.com
7 sify.net
7 regionalnet.org
7 qsc.de
7 progress-energy.com
7 newskies.net
7 home.nl
7 hansenet.de
7 cgocable.ca
7 bluewin.ch
7 arcor-ip.net
7 accesscable.net
7 GeorgiaSouthern.edu
6 vzavenue.net
6 tp-lj.si
6 telekom.at
6 skyriver.net
6 silverserve.com
6 planet.nl
6 ntli.net
6 npgco.com
6 neolane.net
6 ientry.com
6 hinet.net
6 gte.net
6 gingerall.cz
6 comhem.se
6 charterwv.net
6 charterpipeline.net
6 charterpa.com
6 algx.net
6 ad.jp
6 Level3.netComcast should feel right at home with Disney.
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