Which mail and network ports are required?

Direct delivery to recipient mail servers requires outbound TCP 25 allowed by both the VPS provider and firewall. Receiving mail or inbound forwarding also requires inbound TCP 25. Permit any additional submission, relay, DNS, HTTP or HTTPS paths required by your selected topology.

A relay client using a separate compatible hub sends over outbound HTTPS/443 and does not need its own local Postfix stack or inbound SMTP listener. Opening a port alone does not establish a secure mail service.

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