ppp sendmail problem!!!

Dear all

I am having a problem in sending mails using Dialup (PPP) connections.

I will explain the setup.
 I had configured a mail   server in my office using Linux sendmail and 

Redhat 9, which handles the complete intranet and mails to internet.Now 

i am facing a problem  in which i am not able to send mails to very few 

domains, like  rediffmail.com



for eg:

root# telnet mail1.rediffmail.com 25
Trying mail1.rediffmail.com
Closing connection of mail1.rediffmail.com
Trying mail2.rediffmail.com
Closing connection of mail1.rediffmail.com
Trying mail2.rediffmail.com
Closing connection of mail2.rediffmail.com
...
Connection Closed by Foreign host

root# 

This is the problem. But i am able to telnet to ther hosts like
yahoo.com, hotmail.com etc.

The solution i found in the internet was

either recompile kernel with CONFIG_INET_ECN = N or
sysctl by  /proc/sys/net/ipv4/tcp_ecn with 0 which
  allows ECN support to be disabled at runtime.Redhat 2.4 kernel
comes with ecn enabled , but 2.2 kernel works fine.


I tried out the above options but i was not able to send mails to
rediffmail.com. My server is refusing this on a dialup connection
through VSNL. But i am able send mails to rediffmail.com from the same
setup
with a DSL or Cable internet connection.

Thanks in advance.


Regds

Narayanan Subramaniam
Techncial Head
Focuz Infotech, Cochin, India