ubuntu 10.4的postfix安装不了

[3gchinaedu]

安装postfix的时候出现如下提示：
root@3gchinaedu:~# apt-get install postfix
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package postfix is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package postfix has no installation candidate
因为我是新手，请各位帮忙解决一下吧。