aptitude and "Untrusted packages could compromise your system's security."

[Arnau <arnaulist@andromeiberica.com>]

Hi all,

  I've installed a new server and everytime I want to install a new 
package the following message appears:

WARNING: untrusted versions of the following packages will be installed!

Untrusted packages could compromise your system's security.
You should only proceed with the installation if you are certain that
this is what you want to do.


In other servers I have appeared something like "The following 
signatures couldn't be verified because the public key is not available: 
NO_PUBKEY 8899AABBCCDDEEFF" and I know how to fix it.

How can I remove the "WARNING: untrusted versions of the following 
packages will be installed!..."?


Thanks
--
Arnau