How can I judge the mail has been read?

Hello,everyone!I face a problem when I programming,I'd like to ask for you:
I enter the mailbox from webpage,and read some mails,so I'd like to remain the mail in my mailbox.(in fact I use some commerce Email client software like FOXMAIL,it doesn't download this kind mail),

as the picture :http://p.blog.csdn.net/images/p_blog_csdn_net/ZhengKarl/249778/o_ScreenCopy.JPG

shows,the mail has been read is been distinguished from the mail has not been read in mail server website.I use pop3 protocal to download the emails,they seem the same...........

SO when I programming, how can I judge whether the mail has been read from webpage?...?If you kown,pls email me:Karlzheng@126.com.I'll appreciate very much.Thank you very much!

BestRegards!

KarlZheng