How do I automatically forward my mail to another system?

Because open proxies are often implicated in abuse, a number of methods have been developed to detect them and to refuse service to them. IRC networks with harsh usage policies automatically test client systems for known types of open proxies. Likewise, a mail server may be configured to automatically test mail senders for open proxies, using software such as proxycheck. Increasingly, mail servers are configured out of the box to consult various DNSBL servers in order to block spam; some of those DNSBLs also list open proxies.