Most of u must be knowing this , but for those who do not here it goes,
Let me tell u why it is important to know the ip address of the sender who sends u any mail ,
If u have heard about fake mails , i guess u probably know why it is important , fake mails as the name says are sent by people with an email address which is not their own ,
for example , a person can send mails to others with ur yahoo id without even knowing ur password ,
this is called fake mailing and one of the ways or may be the only way to protect urself against such mailers is knowing the ip address of the sender,
for that sign into yahoo mail with ur yahoo id , and u will find "options" on the right hand corner of the page ( also if u click on "mail" on the top left hand side of the page a drop down menu appears , u find options under that as well)
under that click on "general preferences" and then scroll down u will find "messages" under that select " show all headers on incoming messages"
thats it , save it and now when ever u receive any mail u can check out if that ip address belongs to someone u know ,
Hops its of some help