sendmail, 4th Edition

Build a Milter

A Milter is a program that listens on a socket. It receives each email message interactively on that socket from sendmail and receives each message in pieces. The sendmail program first offers the connection information, and the Milter can take it for review or decline it. If it accepts, it will screen that information and either reject the message based on its review or allow the message. Then the next piece of the message is offered and reviewed in the same manner. The order of the review is:

Connect

Review based on the IP address and hostname of the connecting site

Greeting

Review based on the hostname given as part of the SMTP HELO or EHLO command

Sender

Review the envelope sender as supplied as part of the SMTP MAIL From: command

Recipient

Review the envelope recipient as supplied as part of the SMTP RCPT To: command

Headers

Review the header portion of the email message

Data

Review the SMTP DATA command

EOH

Signals the end of the header portion of the message

Body

Review the message body, which can include MIME-encoded portions

EOM

Signals the end of the body portion of the message

The program must quickly (within the timeouts defined by the X configuration command) parse the message pieces and decide whether the message should be accepted or rejected. The program then advises sendmail of its decision, using the libmilter API.

The sendmail source distribution includes a library and sample program that you should use to create your own Milter program. Look in the directory libmilter ...