proxy-nginx/mailu/overrides/location_fix.conf

location / {
    rewrite ^()$ $1/ permanent;
    rewrite ^/(.*) /$1 break;
    
    include /etc/nginx/proxy.conf;
    auth_request /internal/auth/user;
    error_page 403 @sso_login;
    proxy_pass http://\$webmail;
}