In some environments, it makes no sense to install SpamAssassin on the mail server. For example, the mail server may be underpowered to perform content-checking. Or perhaps users have widely ranging preferences for how much (or indeed whether) spam-checking should be performed, and they may not have accounts on the mail server or any convenient way of configuring their preferences. In these environments, one way to provide those users who want the power of SpamAssassin with spam-checking is to help them install a SpamAssassin POP proxy.
Many more POP proxies are available than IMAP proxies, primarily because IMAP is a much more complex protocol and doesn’t require that messages be downloaded to the client. At the time of writing, no freely distributed SpamAssassin IMAP proxies for Windows clients were available.
In addition, most extant proxies call
SpamAssassin through the Perl API to avoid having to run the
spamassassin shell script or a persistent
spamd daemon. Because the Perl API will change in
SpamAssassin 3.0, proxies written for SpamAssassin 2.63 are unlikely
to continue to work until they are upgraded.
Proxy software is middleware. A proxy receives connections from a client and relays them to a server, intercepting all communication in each direction. Application proxies have been used to pierce smart holes in strong firewalls, to cache frequently accessed data, and to perform a variety of other functions.
Logically, POP ...