Ports that need to be open on firewall for Edge Server subscription with Hub Server to function properly:
For Inbound traffic:
SMTP - TCP port 25 (from Internet)
SMTP - TCP port 25 (from Edge server to Hub server on internal network)
For Outbound traffic:
SMTP - TCP/UDP port 25 (from Edge to Internet)
SMTP - TCP/UDP port 25 (from Hub to Edge server)
LDAP for EdgeSync - TCP port 50389 (from Hub to Edge server) Secure LDAP for EdgeSync - TCP port 50636 (from Hub to Edge server)
Since Edge server needs to communicate with Hub server it is important that it can resolve Hub transport servers by FQDN and Hub transport servers must be able to resolve Edge servers by its FQDNs.
To accomplish this you need to either open 53 (DNS) port and configure internal network adapter to use internal DNS but as a security precaution I would suggest to enter DNS records for Edge servers on local DNS manually and to fill hosts file on Edge servers with FQDNs for Hub transport servers.