I'm having the exact same issue
- Bug#840850: mutt: Mutt can't find S/MIME key to ... Fabien Wernli
- Bug#840850: mutt: Mutt can't find S/MIME ke... Fabien Wernli
- Bug#840850: mutt: Mutt can't find S/MIME ke... 840850bugs . debian . org
- Bug#840850: mutt: Mutt can't find S/MIME ke... Kevin J. McCarthy
