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 GPG Lacre (Portuguese for a 'wax seal') is a content filter for Postfix that automatically encrypts unencrypted incoming email using PGP or S/MIME for select recipients. For an overview of the project, please see the [README](https://git.disroot.org/Disroot/gpg-lacre/src/branch/master/README.md).
 
+# Useful documentation
+* [Postfix After-Queue Content Filter](http://www.postfix.org/FILTER_README.html).
+
 # Porting to Python 3.x
 * Milestone: [Python3 rewrite](https://git.disroot.org/Disroot/gpg-lacre/milestone/24).
 * See: [notes on source code analysis](https://git.disroot.org/Disroot/gpg-lacre/wiki/SourceCodeAnalysis).