Stalwart mail

Correct you need a separate subdomain.

services:
  sliding-sync:
    image: ghcr.io/matrix-org/sliding-sync:v0.99.1
    restart: unless-stopped
    ports:
      - 8881:8881
    environment:
      - SYNCV3_SECRET=yoursecret
      - SYNCV3_SERVER=https://newsub.domain.com/
      - SYNCV3_DB=user=syncv3 dbname=syncv3 sslmode=disable
        host=sliding-postgres password=secret
      - SYNCV3_BINDADDR=0.0.0.0:8881
    depends_on:
      - sliding-postgres
  sliding-postgres:
    image: postgres:16-alpine
    restart: unless-stopped
    environment:
      - POSTGRES_USER=syncv3
      - POSTGRES_PASSWORD=secret
      - POSTGRES_DB=syncv3
    volumes:
      - sliding_db_data:/var/lib/postgresql/data
volumes:
  sliding_db_data: null
networks: {}

This is my sliding compose file, and this is the Caddyfile part just for sliding:

newsub.domain.com {
        encode gzip
        header /.well-known/matrix/* Content-Type application/json
        header /.well-known/matrix/* Access-Control-Allow-Origin *
        respond /.well-known/matrix/server {"m.server":"newsub.domain.com:443"}
        respond /.well-known/matrix/client {"m.homeserver":{"base_url":"https://newsub.domain.com/"},"org.matrix.msc3575.proxy":{"url":"https://newsub.domain.com/sliding-sync"}}
        reverse_proxy /_matrix/* http://192.168.1.1:8008/ {
                transport http {
                        compression on
                }
        }
        handle_path /sliding-sync/* {
                reverse_proxy http://192.168.1.1:8881/
        }

I got exactly this config working. The compose file wasn’t the problem. I had to fiddle around with my caddy proxy setup. Do you want my settings? Or do you use something other than caddy?