Can't send email attachment behind linux router and D-Link router

I have a linux router with 2NIC plus with iptables firewall and snort....eth0 is connected to my ADSL modem...and eth1 connected to my DI-714P+ router at WAN port by using cross cable. DI-714P+ is D-Link wireless router.

1-My problem is...my wireless client cannot send email with attachment...
2-I try to connect directly from eth1 to a single pc ( XP ), and try to send email with attachment using Outlook Express...it succesfull..
3-When I try to connect the ADSL modem to D-Link router dirextly and try to send email attachment from wireless client...it succesfull...
3-I think my firewall is ok because i already open port for outgoing email in the firewall setting like below:
OPEN_TCP="20 21 110 143"
OPEN_UDP="20 21 110 143"

I use arno-iptables-firewall-1.8.2a-stable.tgz

Thank you.
ftpfreakAsked:
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ahoffmannConnect With a Mentor Commented:
check both packetfilter: iptables on linux and whatever you have in d-link
use
  tcpdump -l -n -i eth0
to get a glue what's going on, if you donÄt see connections to port 25, it's most likely a firwall (d-link) problem
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ftpfreakAuthor Commented:
help!
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ahoffmannCommented:
does this only happen if the firewall (iptables) is active? then the missing port 25 might be the problem
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ftpfreakAuthor Commented:
yeah...happen when iptables firewall is active...the port 25 you mean i need to allow tcp and udp for port 25?...i try
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ahoffmannCommented:
tcp port 25 should be sufficient
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ftpfreakAuthor Commented:
now my client can send email attachment after i do this:

linux router -(connect from eth0 to vpn router WAN port)- > d-link vpn router -(connect from Lan at vpn router to WAN port)-> dlink wireless router -> wireless client

but when i use this, my client cannot send email with attchment...

linux router - >d-link wireless router -> wireless client

i try to reset the wireless router and just use the basic configuration...the problem is still the same...

what do you think?
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ftpfreakAuthor Commented:
ok thanks....is  it normall when we do a linux router...eth0 connect to internet and eth1 is connect to d-link wan port by using cross cable...the internet at d-link will be not stable...up and down,,
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