Lacre, which means “a wax seal” in Portuguese, is an add-on for the Postfix mail server that makes it possible to automatically encrypt incoming email with user-provided public key. This means, that even if the sender had written their message in plain text, right before delivering their message to recipient's inbox Lacre encrypts it with recipient's key 1).
To be more specific, it is a content filter written in Python. It's a fork of gpg-mailgate, which in turn was based on work of several contributors.
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