Many external mail servers are connected to and managed via a POP3 or Post Office Protocol system. This traditionally includes most Internet Service Providers (ISPs) although it is a service increasingly offered by the web-mail based services.
Anti-Spam supports POP3 services via a system known as a transparent pop3 proxy, that is to say, any traffic originating on a local network and accessing the Internet via an enabled Anti-Spam will have any traffic on the POP3 port, port 110, intercepted and analyzed by Anti-Spam.
In these cases, little more is required than to install Anti-Spam somewhere in the traffic path between the client machine and the POP3 server and enable the transparent POP3 proxy.
Note that due to the manner of the POP3 protocol, Anti-Spam is only able to offer a limited array of options for unsolicited or malware infected email. It is not possible to completely discard any such messages when processing POP3 communications in-line. Therefore, the options are limited to those that will either amend the content of the email, or, in the case of malware, strip any offending attachments or content.
Note: In some situations, a mail server, such as Microsoft Exchange, may be using POP3 for mail retrieval since the POP3 protocol is not strictly limited to client delivery. In these scenarios, Anti-Spam’s transparent pop3 proxy would allow for email to be processed on route to the mail server.