Restrict outlook traffic through Squid

Dear All

I have redhat linux 9, configured transparent squid proxy along with iptables firewall. All my users acceessing their mails through outlook and my mail server is hosted with third party. We have pop3 and smtp address using that we are sending / receiving mails through outlook.

I want to restrict certain users to send mail only to specified domain like xyz@llspl.com should send mail xyz@ing.com. Is’t possible through proxy? if yes, then kindly let me know the procedure.

Regards
Sakthi