[CentOS] Updating - CentOS Repository

Hello,
I was wondering if any of you know why the CentOS repository are not 
updated? for example, i just heard about the bugs of the version <=1.4.4 
of Squirrelmail and on their website there is already a stable packet 
1.4.5, but in the CentOS repository the only available version is the 
1.4.3 9a

Any ideas?

Another thing, im new using yum so i have a question, for example if i 
put "yum install squirrelmail1.4.5" but i have the version 1.4.3 already 
installed in my system, what happens? does the yum override the old 
version? what happens with the config files?