OpenDKIM provides a milter as well as a standalone library. Milter support was added to Postfix 2.3.0 released July 12, 2006. The application also features a Milter interface for sendmail and on-demand scanning. The Milter library included with Sendmail uses POSIX threads to allow multiple concurrent filters. The infrastructure provided by mimedefang.pl follows the Milter API reasonably closely, but not exactly. Milter has undergone substantial improvements since its first official release. Milter was first officially introduced to Sendmail 8.12.0 released on September 8, 2001.