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
------DB0A8603FEC143B828778CD72C7611C6
Date: Wed, 19 Aug 2020 16:01:30 +0200
From: Marcel Herrguth via Zammad Helpdesk <mail@example.com>
To: Marcel Herrguth <user@example.net>
Message-ID: <20200819140129.XYZ371386881337@example.com>
In-Reply-To:
References:
Subject: MH Verification
Mime-Version: 1.0
Content-Type: multipart/mixed;
boundary="--==_mimepart_5f3d30ba227d5_72734926d881336e5";
charset=UTF-8
Content-Transfer-Encoding: 7bit
----==_mimepart_5f3d30ba227d5_72734926d881336e5
Content-Type: multipart/alternative;
boundary="--==_mimepart_5f3d30ba225cc_72734926d881334b7";
charset=UTF-8
Content-Transfer-Encoding: 7bit
----==_mimepart_5f3d30ba225cc_72734926d881334b7
Content-Type: text/plain;
charset=UTF-8
Content-Transfer-Encoding: base64
[...]
The file 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.