We've currently installed a new Exchange Server 2013 (NOC 1 internal corp mail behind firewall) however currently use an external mail Server (NOC2) IMAP POP3/SMTP for clients to authenticate and download and relay (SMTP) their e-mail. I'm looking for advise on how to effectively maintain the current Server performing primary scanning/anti virus & RBl filtering and relay, post scanned e-mail to our internal corporate Exchange 2013 server. Also, would like, if at all possible, best practices for over-all security hardening of IIS & Exchange.
Thank you in advance,
William