Hi
You have an option straight from NS distor. Proxyy for your emal server .
Secound option for you is filter your smtp traffic and redistrict to your own gateway.
NS can do the job for you in “old fashion” . I think is not god way to double “security checks /antivir” no point if you will do that on different host with the same policies.
To keep all this going you need a poith to check you can name it /anty AV or SPAM. this is what i will do -> domain -> rspamd-> don’t be e cunt when a millions of people get the same respond-> then if you relay have more issue , build av mail gateway, (https://www.proxmox.com/en/proxmox-mail-gateway) , what is your question? do you believe to build system never get any abuse/spam?
no point