Kept headers.

Bcc:
Cc:
Content-Type:
Content-Transfer-Encoding:
Date:
From:
Message-ID:
In-Reply-To:
MIME-Version:
References:
Reply-To:
Resent-Bcc:
Resent-Cc:
Resent-Date:
Resent-From:
Resent-Message-ID:
Resent-Sender:
Resent-To:
Return-Path:
Sender:
Subject:
To:

Received: is blacklisted, as it isn't critical to message understanding, and as it wastes lots space.

Example 1 (1101 bytes saved, 23.8%): Example 2 (6163 bytes saved, 17.3% compressed):