Milter refers to the Sendmail Content Management API and also to software written using this API. Milter provides a plug-in interface for third-party software to validate, modify, or block messages as they pass through the MTA. In sendmail, milter consists of support code in sendmail itself and a separate milter library. Filter authors link their filters against this library to produce a server. Sendmail is then configured to connect to these milter servers. Messaging Server provides a library that emulates the sendmail side of the milter interface. Consequently, milters written for sendmail can also be used with Messaging Server. The milter server can run in a variety of configurations. It can run on a separate system of its own, on the same system as Messaging Server, in a single system deployment, or in a two-tier deployment. Messaging Server also supports connecting to multiple milter servers.