Communication Between an MTA and Milter

Filter applications run as separate processes outside the Sendmail address space. The benefits of filter applications running as separate processes are as follows:

Each filter can communicate with multiple MTAs at the same time over local or remote connections, using multiple threads of execution. Figure 1-1 “Communication Channel Between MTAs and Filters” illustrates a network of communication channels between the filters, its MTAs, and other MTAs on the network.

Figure 1-1 Communication Channel Between MTAs and Filters

The Milter library (libmilter.a) implements the communication protocol. It accepts connections from various MTAs and passes the relevant data to the filter through callbacks.