Question : Can't send email to certain domains

For only a select few domains, I am having trouble getting email from our domain through to their domain.

We are using an Exchange 2000 server on a W2k Server box.

If I send an email, with or without an attachment and from any domain user, to [email protected], it sits in the SMTP queue for a few days, and then finally generates an NDR saying Could not deliver the message in the time limit specified.  This happens for a few customer domains, as well as Comcast.net.

In the Message Tracking Center for that email, it shows repeated SMTP: Started Outbound Transfer of Message and then Message transferred to  through SMTP  - every 10 minutes or so.

Ive checked multiple spam/rbl lists, and we dont appear on them.

Reading through posts here with similar symptoms lead me to check on the reverse lookup.  From a lookup on network-tools.com, I get this if I type in ourmailserver.domain.com:

DNS servers
ns2.isp.net
ns.isp.net

Answer records
ourmailserver.domain.com      1      A      123.45.56.789 (our actual public ip appears here, not this one)      3600s

Authority records            
ourdomain.com      1      NS      ns2.isp.net      3600s
ourdomain.com      1      NS      ns.isp.net      3600s


Any ideas??  Thank you in advance!                        
            

Answer : Can't send email to certain domains

Ok lets start...
So as i understand.. bigcustomer.com is one of the domain where you are not able to send mails Right?

do this...on command prompt
nslookup
set q=mx
bigcustomer.com

.... does it give you the MX of that domain with the IP ?
Please paste the results here,...

then do telnet to MX of bigcustomer.com

telnet MX/IP 25
- what does it do...? connects and gives you banner>?  or doesnt connect?
if doesnt connect may be you are blocked some wehre..

let me know the results and i wil help you further./..

:) Suraj