Basic proxy service

The NGINX mail proxy module was originally developed for FastMail. They needed to provide a single IMAP endpoint for their users, while hosting the actual e-mail account on one of the number of upstream mail servers. Typical proxying programs of the time used the classic Unix forking model, which meant that a new process was forked for each connection. IMAP has very long-lived connections, which means that these processes would stay around for a very long time. This would then lead to very sluggish proxy servers, as they would have to manage these processes for the lifetime of each connection. NGINX's event-based process model was a better fit for this type of service. As an e-mail proxy, NGINX is able to direct traffic to any number of mailbox servers where the actual e-mail account is hosted. This provides the ability to communicate one endpoint to customers, while scaling the number of mailbox servers up with the number of users. Both commercial and opensource...