For this problem
warning: database /etc/postfix/virtusertable.db is older than source file /etc/postfix/virtusertable
After you edit file /etc/postfix/virtusertable you MUST type the command
with this command you will solve this problem.
You can create the file /etc/postfix/access, and after you will create this file or modify do NOT forget to run command:
For problem with yahoo check if your port 25 in your server mail is filtred or DROP on chain FORWARD in iptables.
Check wich user send more mail, with you, or spam.
Check your ip if it's blacklisted, see this link: http://www.mxtoolbox.com/SuperTool.aspx