magento's email function doesnot work at LAMP

following the document below

http://library.linode.com/web-applicati … an-5-lenny">http://library.linode.com/web-applications/e-commerce/magento/debian-5-lenny

first,I do "getting started "

second,I do " LAMP guide"

third,I do "Magento on Debian 5 (Lenny)"

they are wonderful documents,now I have successfully insalled magento1.7 ,and it run well,

I can sign up,order ,manage order etc. but now I found a problem,it's email function doesnot work,

as when sign up,order,my email box didnot receive any email,and at contact us page,also cannot send out email.it show "Unable to submit your request. Please, try again later".

my websites is www.yesshopping.biz

I guess I need to setup a email server,but which document should I follow for magento?