Hi Brian, Any server can be a security threat, whether on windows, linux or mac. But windows servers are more vulnerable compared to mac and linux. There are some minimum standards of practice required to make a server safe, this is more explicit on linux than windows. Thats why most of all critical services on the web are almost entirely run on linux today. I dont see where the conflict arises if the mail server is on windows, koha on linux and others are on mac. In my environment, we run our mail server on linux CentOS, proxy is on debian linux, koha is on ubuntu linux, dns is on windows server.