admin.py

class MessageEventAdmin(admin.ModelAdmin):
    #...
    def response_change(self, request, obj):
        if "_send_email" in request.POST:
            message_text = str(obj.msg_text)
            if obj.msg_html:    
                email = EmailMultiAlternatives(
                        subject=obj.msg_subject,
                        body=message_text,
                        to=[obj.msg_envelope_recipient],
                        from_email=str(obj.msg_envelope_sender),
                        reply_to=[str(obj.msg_envelope_sender)]
                        ) 
                email.attach_alternative(str(obj.msg_html), "text/html")
            else: # no HTML, just text
                email = EmailMessage(
                        subject=subject,
                        body = message_text,
                        from_email=str(obj.msg_envelope_sender),
                        to=[obj.msg_envelope_recipient],
                        reply_to=[str(obj.msg_envelope_sender)]
                        )
            email.send(fail_silently=False)
        return super().response_change(request, obj)