Is it possible to run a mailserver on a nginx layer 4 server?

I've just created a layer4 nginx server, which is used as a load-balancer for a kubernetes cluster.

I know it is a very basic question (and probably I'll get a lot of downvotes for this), but is it possible to install and run a mail-server on the same machine? Is the mailserver traffic just running on different ports and therefore it shouldn't be a problem?

My nginx config looks like this:

user www-data;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;

events {
    worker_connections 768;
}

stream {
    upstream http {
        server 123.45.67.170:80;
        server 123.45.67.171:80;
        server 123.45.67.172:80;
    }
    upstream ssl {
        server 123.45.67.170:443;
        server 123.45.67.171:443;
        server 123.45.67.172:443;
    }
    server {
        listen 80;
        proxy_pass http;
    }
    server {
        listen 443;
        proxy_pass ssl;
    }
}

http {
    sendfile on;
    tcp_nopush on;
    types_hash_max_size 2048;

    include /etc/nginx/mime.types;
    default_type application/octet-stream;

    ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
    ssl_prefer_server_ciphers on;

    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;

    gzip on;

    include /etc/nginx/conf.d/*.conf;
}

and I think I have to add something like this:

mail {
    server_name mail.test.com;
    auth_http   http://127.0.0.1:8000;
    xclient off;
    server {
        listen     3333;
        protocol   smtp;
        smtp_auth  none;
    }
}

But if this is correct, should I install the mailserver on this machine itself or should I use a mailserver, which is running in the k8s cluster?