emails bouced to bulkmail folder

[RYAN vAN GINNEKEN]

Need some info on the what the large email severs ie Yahoo and Hotmail 
use to determine if email is spam.  I run a small email server with 
about 30 clients and when i send email to yahoo it gets bounced to the 
bulk mail folder like in my maillog below. 

Jul  5 15:30:19 v22 postfix/smtp[14316]: starting TLS engine
Jul  5 15:30:19 v22 postfix/smtp[14316]: C52C23C9: 
to=<vanginnekenr at yahoo.com>, relay=mx2.mail.yahoo.com[64.156.215.8], 
delay=1, status=sent (250 ok dirdel)

Read somewhere that they do a reverse lookup on your ip address for a 
host name.

here is my info for reverse lookup for the ip 209.115.173.22

Non-authoritative answer: 22.173.115.209.in-addr.arpa name = 
mail1.computerking.ca. Authoritative answers can be found from: 
173.115.209.in-addr.arpa nameserver = toad.spots.ab.ca. 
173.115.209.in-addr.arpa nameserver = cheetah.spots.ab.ca. 
toad.spots.ab.ca internet address = 209.115.174.3 cheetah.spots.ab.ca 
internet address = 209.115.174.2

looking forward for computerking.ca

Server: ns1.kloth.net Address: 213.133.98.149#53 Non-authoritative 
answer: Name: computerking.ca Address: 209.115.173.22 computerking.ca 
mail exchanger = 10 mail1.computerking.ca. computerking.ca mail 
exchanger = 20 mail1.shoemasters.computerking.ca. computerking.ca mail 
exchanger = 30 mail1.highcoup.ca. computerking.ca origin = 
computerking.ca. mail addr = root.computerking.ca. serial = 2004062001 
refresh = 10800 retry = 3600 expire = 604800 minimum = 3600 
computerking.ca nameserver = ns1.shoemasters.computerking.ca. 
computerking.ca nameserver = ns1.computerking.ca. computerking.ca 
nameserver = ns1.highcoup.ca. Authoritative answers can be found from: 
computerking.ca nameserver = ns1.highcoup.ca. computerking.ca nameserver 
= ns1.shoemasters.computerking.ca. computerking.ca nameserver = 
ns1.computerking.ca. mail1.computerking.ca internet address = 
209.115.173.22 mail1.shoemasters.computerking.ca internet address = 
68.144.231.38

Does it have something to do with the fact that my ISP is running the NS 
for reverse lookup and my own nameservers take care of things for 
forward lookups.   Should i see if i can run my own nameservers for 
reverse lookup as well.  What is it that yahoo does not like about my setup.

please someone help me i do not know what could be wrong and have 
switched ISP's to try and solve this problem but so far it has not worked.