apt install 的 server??

[kennyk]

# apt-get install postfix
Reading Package Lists... Done
Building Dependency Tree... 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
W: Couldn't stat source package list http://security.debian.org stable/updates/non-free Packages (/var/lib/apt/lists/security.debian.org_dists_stable_updates_non-free_binary-i386_Packages) - stat (2 No such file or directory)
W: You may want to run apt-get update to correct these problems
E: Package postfix has no installation candidate

apt 安裝有更好的 server 嗎?
請各位大大提供謝謝!!

[kennyk]

補充:
OS為 debian-31r0a