At that point it's just "why communication with the outside is a risk to your organization". This article presents an attack vector unique to HTML emails, but

At that point it's just "why communication with the outside is a risk to your organization". This article presents an attack vector unique to HTML emails, but most attacks over email can be easily adapted to work over WhatsApp, Slack, Jira, Zoom, or whatever else people use to communicate with the outside world.