Because SMTP Proxies can be implemented using lighter-weight programming techniques such as asynchronous IO (for example nginx, Node.js, Netty and Libevent), thousands of connections can be juggled using the same resources as a much smaller number of connections in the context of a full-blown store and forward email server.
Similar environments written in other programming languages include Node.js for JavaScript, Twisted for Python, Perl Object Environment for Perl, libevent for C and EventMachine for Ruby.