Sample Filter Pseudocode

The following pseudocode describes the filtering process from the perspective of a set of N MTAs, each corresponding to an SMTP connection.

For each of N connections
{
   	For each filter
       		process connection/helo (xxfi_connect, xxfi_helo)
MESSAGE:For each message in this connection (sequentially)
   	{
       		For each filter
                			process sender (xxfi_envfrom)
       		For each recipient
       		{
                			For each filter
                         				process recipient (xxfi_envrcpt)
   		    }
   		    For each filter
   	    	{
          			For each header
                   				process header (xxfi_header)
          			process end of headers (xxfi_eoh)
          			For each body block
                   				process this body block (xxfi_body)
          			process end of message (xxfi_eom)
    		    }
	    }
	    For each filter
    	         	process end of connection (xxfi_close)
}

The callbacks within parenthesis are placed beside the processing stages in which they are called. If a callbacks is not defined for a particular stage, the filter application can bypass that stage. The filter application can abort processing at any time during a message, in which case the xxfi_abort() callback is invoked and the control returns to MESSAGE.

Sendmail contacts the filter applications in the order defined in the Sendmail configuration file.

To write a filter application, you must invoke different callbacks to process relevant parts of a message transaction. The Milter library then controls all sequencing, threading, and protocol exchange with Sendmail.

Table 3-1 “Milter Callbacks Related to an SMTP Transaction” outlines the control flow for a filter process and denotes different callbacks invoked during an SMTP transaction.

Although Table 3-1 “Milter Callbacks Related to an SMTP Transaction” denotes only a single message, multiple messages can be sent in a single connection. The remote host or Sendmail can abort a message and connection anytime during the SMTP transaction. If the abort occurs during a message processing (that is, between the MAIL command and the final . command), the filter application calls the xxfi_abort() API. The filter application calls xxfi_close() any time when the connection closes.