Whilst I cannot be sure as to the meaning of the original poster, I thought that it might be interesting for me to interject at this point with some detail about some work that I have done with Perl and the Sendmail MTA. Using the mail filter (milter) interface of Sendmail and building upon the Sendmail::PMilter module (which I have tertiarily been involved in the development of), I rebuilt the entire architecture of the Bluebottle challenge-response mail service (http://www.bluebottle.com).

This service, when I first became involved with Bluebottle in early 2002, was built in a queue-processing fashion, employing a custom local mailer with Sendmail to dump inbound mail into a queue processing directory, from whence a daemon process would pick up the mail and process it according to user delivery preferences. This approach however proved to be quite limiting with the service eventually being crushed under the weight of a joe-job spam attack necessitating a better through-put time for mail messages.

After spending some time on analysing system bottle-necks and identifying potential system improvements, I set about rebuilding a version of the system which employed the milter interface of Sendmail - This interface allows for external mail filter code to access and respond to elements of the SMTP transaction as they occur and thereby provided the perfect level upon which to enact user delivery preferences. Further information about the milter interface can be found at http://www.milter.org/milter_api/index.html.

Relaunching the challenge-response component of the Bluebottle mail service employing the milter interface for mail message inspection and application of user preferences in January 2004 led to phenomenon growth of the service, which the mail service, with the revised system architecture scaled with most admirably, grew from around 8,000 users at the start of year to almost 80,000 users at the years end. The total number of messages handled by this revised mail architecture is estimated to be between 35 to 45 million messages.

This is how I have used Perl with a MTA (Sendmail) ...