Enable Port 25 On-Premise

I am running my own physical mail server on-site and port 25 is currently blocked. I need it opened and correctly routed so the server can both send and receive e-mails without relying on external relays... (Budget: $2 - $8 USD, Jobs: Cisco, Firewall, Linux, Network Administration, Network Security, SMTP, System Admin, Troubleshooting)