Dear Sir;

    This Side Rahul  and I want to know that How a mail flow between a sites ?

Hi Rahul,

For your better understanding about mail-flow in exchange 2013, I would like to refer at below few resources which would be a good assistance for you. Please go through :

http://technet.microsoft.com/en-us/library/aa996349%28v=exchg.150%29.aspx

http://www.msexchange.org/articles-tutorials/exchange-server-2013/planning-architecture/exchange-2013-mail-flow-part1.html

http://msexchangeguru.com/2012/08/09/e2013-mailflow/

I hope, it will help you lot to understand and clarify your concern in depth.