Port 25: This port continues to be used primarily for SMTP relaying. SMTP relaying is the transmittal of email from email server to email server. In most cases, modern SMTP clients (Outlook, Mail, Thunderbird, etc) shouldn't use this port. It is traditionally blocked, by residential ISPs and Cloud Hosting Providers, to curb the amount of spam that is relayed from compromised computers or servers. Unless you're specifically managing a mail server, you should have no traffic traversing this port on your computer or server.
Routing all mails to a securehost :
These instructions assume the postfixconfig files live in /etc/postfix/main.cf
In /etc/postfix/main.cfadd the line:
relayhost = mailout.domainonair.com
Routing all mails for a specific domain to a securehost :
Add a line to /etc/postfix/transport:
generate a postmap file :
To use the transport file, add or edit a line in /etc/postfix/main.cf:
transport_maps = hash:/etc/postfix/transport
Restart Postfix and all mail. The mail for selected domains should go trough the securehost.