You QNAP server is most likely running behind a NAT - meaning it will have a local IP address like 192.168.x.x or 10.x.x.x. Therefore, you will have to open the ports in your firewall to allow incoming traffic for port 25 to come to your QNAP server. Once port forwarding is done on your firewall, servers on the Internet will:

• Resolve the public IP address of your domain by querying the MX and then the A record
• They will try connecting to port 25 on your public IP address
• Your firewall will take that request and forward it to port 25 on your LAN IP where QNAP is running