The company requires all external email to go through our own internal email filtering system. When configuring an email action I can select "External email and attachment" which uses an external email service configured by Nintex. I need to prevent users sending email externally either by preventing the selection of this option or by having our domain restricted to prevent the external email service. I have tested this and currently our domain is prevented from sending email attachments (which is good). How can I prevent external email?