Here is a tutorial how to hack email ids thru brute force,its not so much efficient process because it takes a lot of time .... anyhow i thought to share it with u guys atleast it will give u some knowledge ....
Follow these steps :

1.Download Brute Force Software .
http://www.hoobie.net/brutus/brutus-download.html
2.Excract on desktop and lets start/ ok run it

3.To hack email in target bar write:
pop.mail.yahoo.com

4.Type most be: POP3

5.Put connections to 60 and timeout to 60

6.Make sure you check a ''Single User''

7.Then when you check that to single user write in that bar you victims yahoo acc ID / explanation .. like googleboy@yahoo.com his ID will be googleboy .. write that

8.Password mod put on: ''Brute Force''

9.Then click ''Range''(a new window will pop up)

10. Then make sure you put ''Min Lenght'' 6 and ''Max Lenght'' to 16

11.Then put custom range .. there will be default Abcd.. etc and some numbers ..

12.Click OK

if u have any problem then ask here i will try to solve it

and as i have said earlier it takes a lot of time.it actually crack the password.

It really takes time ..... Its just like a cracker .... I think u have seen in movies to break the digital lock they try all the numbers quickly and the correct number opens it ........ just like that in brute force u have a list of passwords and the software try all these password line wise .... so it may take a lot of time for ur desired password to come

** If the password is alphanumeric then its almost impossible ......