Nope sorry, impossible to help you.
My guess is that the mail is empty at this moment.
This is how a smime signed mail looks like (right from the first line below the headers).
This is an S/MIME signed message
Date: Wed, 19 Aug 2020 16:01:30 +0200
From: Marcel Herrguth via Zammad Helpdesk <firstname.lastname@example.org>
To: Marcel Herrguth <email@example.com>
Subject: MH Verification
smime.p7m is simply the ceritificate used for verification. And technically has nothing to do with the content living in the source of a mail. (Apart the signage). If above you provided is everything of the body, Zammad has nothing to display which is why “no visible content” will appear to indicate exactly that.