Hi,
I have configured a mailman3 mailinglist to not modify the headers and body (e.g. no footer, no subject prefix, etc) to avoid breaking DKIM signatures. This worked well, until I encountered an email where the DKIM signature was still broken. After some debugging, the problem is caused by an attachment, where the mime header is changed by mailman3.
The original mail contains this:
--------------nYYjj8vJ8CNKuoHJZKVOQh20 Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet; name="filename.xlsx" Content-Disposition: attachment; filename="filename.xlsx" Content-Transfer-Encoding: base64
Mailman3 modified these into this:
--------------nYYjj8vJ8CNKuoHJZKVOQh20 Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet; name="filename.xlsx" Content-Disposition: attachment; filename="filename.xlsx" Content-Transfer-Encoding: base64
The Content-Type header is modified (word-wrapped), and this breaks the DKIM signature.
How can I prevent this from happening and configure mailman to not modify the email?
Jef