As a rule of thumb, you should get rid of integrated antispam (which user interface is quite limiting) and use an external antispam that will handle DKIM, TLS, and mail queues.
This way, you'll offload quite a bit traffic and processing power from Grommunio.
A good solution that comes in mind is Proxmox Mail Gateway.
Hence, you'll fine tune Proxmox postfix instead of grommunio's.
