I was able to send mails using the unix systems those have internal ip 10.10.10.x to one of our public mail servers. Suddenly the messages started bouncing back since 10 days and showing up the error message as:
Diagnostic-Code: SMTP; 550 5.1.8 invalid/host-not-in-DNS return address not allo
I am just using simple sendmail client and the sendmail daemon is not running on them.
I am able to send mails from the same systems to yahoo mail and hotmail without any problem.
Thanks in advance.