Checkbox has no effect: "Attach signature to emails sent from external applications"

The checkbox “Attach signature to emails sent from external applications” has no effect.
EmClient 10.0.3266 alway attaches the selected signature to mails sent from external applications, even with de-selected checkbox.
Our ERP-application genereates an EML-File with inclueded HTML-body with corporate-signature an runs it. Then em-Client attaches a second signature to the mail.
Is there a way do avoid this or is it a bug in current version 10.0.3266?