If your Apache server is set up to operate as a proxy, it is possible for it to be used as a mail relay unless precautions are taken. This means that your system may be functioning as an "open relay" even though your mail server software is actually securely configured.
Use mod_rewrite to forbid proxy requests to port 25 (SMTP):
<Directory proxy:*> RewriteEngine On RewriteRule "^proxy:[a-z]*://[^/]*:25(/|$)" "-" [F,NC,L] </Directory>
To use the Apache proxy as an SMTP relay is fairly trivial, but then so is preventing it. The solution simply tells the server to respond with a 403 Forbidden to any attempts to use it to proxy to a remote mail server (port 25). Other ports, such as HTTP (port 80), HTTPS (port 443), and FTP (ports 20 and 21), which are commonly permitted proxy access, will not be affected.