Problem installing server after installation

[Don]

Hi,
 
Set up a new Fedora Core 3.  At install, I did not install Dovecot IMAP
server but now I'd like to.  I've tried the command:
 
yum -y install dovecot
 
but always get the following error:
public key not available for
//var/cache/yum/base/packages/dovecot-0.99.11-1.FC3.4.i386.rpm 
public key not available for
//var/cache/yum/base/packages/postgresql-libs-7.4.6-1.FC3.1.i386.rpm
 
What are these keys, why do I need them and where can I get them?
I just want my IMAP server.
 
Thanks,
Don

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