Problem with Content-Transfer-Encoding: 8bit

[ikadmm]

I'm running your code to sign emails for my domain with a XmailServer filter.
Sending emails to Gmail works fine when I send simple emails using Thunderbird with Content-Transfer-Encoding: 7bit.

Looking into "Original message" at Gmail we can see DKIM validation:
Authentication-Results: mx.google.com; dkim=pass header.i=@ikadserver.com header.b=eeqn0G1R; spf=pass (google.com: domain of info@ikadmultimedia.com designates 217.182.148.68 as permitted sender) smtp.mailfrom=info@ikadmultimedia.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ikadserver.com; q=dns/txt; s=dkim; bh=69ZeLdowS8WRK8sUT8T02AQuCVJ9YKvyK6Qe02mCy2o=; h=from:subject:to:mime-version:content-type:content-transfer-encoding; b=eeqn0G1RRu6GxTRzXOftY3CIDxlF1z6D1i8dLuLOuHW6lNh7eJtP6llKj1+4buiU5qMfXVbi4kyk pqx8DgL2EYORMiBdEVP9ryTl6ZM1L0eHcHm+1Et2KYWEGlmbylCysHlFhP7WbPbcWn6tnfjENPI1 MxlApNCyj5R7WEyUx9Nm5rkMxmmTJPLELHUiv+cxZ1SFIDS89brRfnJbItuG+L9d0x3+LrliX3YK c53n1eRCmSJOmKoD092HLlGLyP3gXqZkVxxWnK9uS0hCINaCMkSKf77BknmfoTNrWB6GEnSoF/Po TSesf5In3+b3TcjWC2H/jmAjI3Ch4B+OWZP5bg==

Then I reply that message from Gmail and its server send it using Content-Transfer-Encoding: 8bit
When I reply to this email my server adopts 8bit transfer and brokes DKIM signature:

Authentication-Results: mx.google.com; dkim=neutral (body hash did not verify) header.i=@ikadserver.com header.b=CMCapilg; spf=pass (google.com: domain of info@ikadmultimedia.com designates 217.182.148.68 as permitted sender) smtp.mailfrom=info@ikadmultimedia.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ikadserver.com; q=dns/txt; s=dkim; bh=AUXwI81d/1hiIrEU6rdWOlabFkS1YlwSHeJZcNBFfPE=; h=from:subject:to:mime-version:content-type:content-transfer-encoding:in-reply-to:references; b=CMCapilgym520JKpy8h84Z8CZR6UrxKppH099AByiU5VqQQSvhjMKZEOIqHtj4ol8aoLc7LH4L8V EPKlzIm+fIXKFfYNvYINnDDMLgqYjHoJEJYgJFHYArXF5gOEBEJny+zXDRaDdObGE7uIdH5d5byO DN8MPYc0baDo5jjj/P4mXvtD+U7ur0ZAog1AIjrxtR0pxfMOnE0kymsbH8oDD79wYKXETYYqbmfa 8QH06FuLtoNxQqoskdNUKNPEgzjbVGdnvEWsJLub8m8Qlt2yqhTCz0yW3uGuwrOM9obahKA9QTO8 DP2k5osDP2/gMjKEwCHYssglbumNdOydePmYiw==

Is there any option to sign messages based on Content-Transfer-Encoding?
Thanks a lot for your code and time!