On such n/w's I'am able to send mail from xyz@copstop.com to abc@copstop.com using low level SMTP chat from home by telnet to port 25 to one of copstop.com's relay servers facing the internet. Where'as i'am not either of the two.
If it is only a relay server and not an MX then it should not be accepting connections from the internet.
The mail received by abc looks original but shows
from : xyz@copstop.com to: <> undisclosed recipients
in the header and logs.
That is because you did not put the To: header in the DATA part.
Is there a way out where I can stop
mail from: xyz@copstop.com
only when the relay server is accepting mails from other servers.(incoming)
If it is only relaying mail to the internal servers (MX) and not relaying for them,this can fix it: add an entry to /etc/mail/access
From:costop.com REJECT
If it is relaying mail and also acting like an MX, then maybe you can check this page out
http://www.sendmail.org/m4/anti_spam.htm