Strange problem

server, isp

[Djive2005]

I have strange problem.

Have dedicated server and everthing works fine...

Have few ISP in my country.. an all userer on my server can see their pages and read mails

just people on main ISP cant see their website or read emails.

Some domains work other not, then others domains work other not, and some domains dont work.

And with all other ISPs everthing work fine..

Any suggestion to resolve this...

Thanks!