1. Design

          +-----------------+                    +---------------+
(SMTP) ==>|receiver sendmail|==> mail_filter ==> |sender sendmail|==> (SMTP)
 etc.     +-----------------+                    +---------------+     etc.
           sendmail.cf-recv                 sendmail.cf (standard location)

  "standard" means the file should be stored at standard path.
  With this configuration, mails sent from local are not verified.


2. Setup

  a. Generate sendmail.cf-recv with external mailer definition. e.g:

	EXTERNAL_MAILER_PATH=/usr/local/lib/mail/mail_filter.pl
	EXTERNAL_MAILER_FLAG='mDFMuXnP'
	EXTERNAL_MAILER_ARGS='mail_filter $h $g $u'
	EXTERNAL_MAILER_MAXSIZE=100000

     and definitions for dispatching the external mailer. If you want to
     verify all outgoing mails, define as follows:

	DIRECT_DELIVER_DOMAINS=none
	DEFAULT_RELAY='extern:anything'

  b. Generate standard sendmail.cf not to use external mailer and store
     at normal path.

  c. start daemon sendmail with sendmail.cf-recv

