I was setting up a mailserver for a client and couldn't work out why I was continually unable to send messages to our SMTP relay. I had also set up a webmail client which was having similar problems sending email to the mailserver. I'd tried everything I could think of and everything that I found online. I'd even escalated the problem up to our ISP, with no joy.
So would you believe my surprise when I came across a TechNet thread that presented the solution to my problem; something so obscure I never would have thought of it.
It turns out that McAfee's 'Client Protection' component was blocking telnet from accessing the server by IP address and hostname. Once I'd disabled the client protection component I was able to telnet to the local server, the SMTP relay server and my webmail client was able to send emails to the mailserver.
I have to give many thanks to Rich Matheisen for his informed post on TechNet.
