Are you using the SMTP Proxy server or the Regular SMTP server in Xeams? I ask because if you use the Proxy Server, anyone on the Internet trying to Authenticate will actually connect to your Exchange through Xeams.

I recommend switching from SMTP Proxy to regular SMTP Server for inbound emails. Once this is done, your Exchange will completely get disconnected from the Internet and you should not see any attempt to authenticate on your Exchange.

Check http://www.xeams.com/DifferenceSmtpAndSmtpProxy.htm for details about differences between the two servers.