The inline_smtp Perl script was created to allow filtering of incoming mail on a separate server that houses the mailspools without running a full-blown MTA. While it's possible to configure another MTA, e.g. postfix, to relay all mail to a smart host and do filtering through postfix, the queue can get behind easily, and on a busy host thousands of messages can get queued in a few hours causing delays in delivery from a few minutes to several hours. By filtering while redirecting, mail arrives without any delays, and can be filtered by a seperate host adding headers or content reducing the load on the final mail host.
