Originally released in May of 2002, SpamPD is a program used within an e-mail delivery system to scan messages for possible Unsolicited Commercial E-mail (UCE, aka spam) content. It uses an excellent program called SpamAssassin (SA) to do the actual message scanning. SpamPD acts as a transparent SMTP/LMTP proxy between two mail servers, and during the transaction it passes the mail through SA. If SA decides the mail could be spam, then SpamPD will ask SA to add some headers and a report to the message indicating it's spam and why. SpamPD is written in Perl and should theoretically run on any platform supported by Perl and SpamAssassin.
Check the Releases area for latest versions, and see the old Downloads section for older releases (and the "previous-versions" folder for some even more ancient ones). Recently the Debian package version was added to this repo as a branch, and those tags will also show up in the Releases page.
More historic background information is available here.
Linux packages data courtesy of Repology: