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/usr/share/doc/procmail/README.Maildir is in procmail 3.22-25+deb9u1.

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The actual contents of the file can be viewed below.

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This version of procmail supports Maildir folders.

To make procmail to deliver into a Maildir folder, just append
a slash (/) to the name of the maildir folder in your ~/.procmailrc file.
For example, the following rule:

:0
* ^Resent-Sender.*debian-user-request@lists.debian.org
debian-user/

will deliver all mail from the debian-user mailing list to the Maildir
folder "debian-user".