cannot recognize email receiving is reply or new mail recivi

I doing contact system web application using c sharp asp.net

using library mail kit

the problem is when some one reply to my email from outlook 2010 to any email I sent it then on my web application reply id

on debug show null when receiving .

so How any email server like google,Microsoft know that email reached to inbox is new mail or reply for email ?

I already loop for inbox and get emails then show it

How email inbox message reached recognize that received message is new mail message or reply to mail sent before ?

if i test send or reply for any message using outlook it not have any problem

also send receiving and reply work good

problem on my web application asp.net receiving reply to email for second time .

I need any way solve my problem on web application .

receiving emails not recognized on my inbox web application if reply or new message

using mail kit library .

How to solve problem