First of all stating that it's urgent has a very strange effect here on LinuxQuestions. Most users (if not all) just don't react when you state it's urgent because it's only to you that the urgency applies. Since we all are volunteers here nobody jumps up to answer whatever questions you have. Instead we do it on our own time and rhythm.
Second of all, what have you got so far? What configuration do you have running now? Or are you just starting out with the installation? If that's the case then you should look at Google first for answers and post here when what you have fails for reasons that are unclear to you.
That's when LinuxQuestions comes in, providing answers where you cannot find them. Answers, not how-to or step by step guides.
In the meanwhile, if you're just starting out you might have a look at this link:
Sendmail on CentOS 5