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Add p5-Mail-Qmail-Filter: Modules for filtering between qmail-smtpd and qmail-queue

Mail::Qmail::Filter and its submodules are designed to help you filter
incoming e-mails when using qmail as MTA.

You should use it like so:

1. Write a frontend script to configure your filters
2. In the run file for your qmail-smtpd instance, e.g.
   /var/qmail/supervise/qmail-smtpd/run, export
   QMAILQUEUE=path_to_your_frontend_script

In each filter, you may do various things:

- examine and change envelope data (RFC5321.MailFrom and recipients)
- examine and modify the e-mail message (header and/or body)
- "reject" e-mails (or defer them)