Tecadmin published a tutorial showing you how to configure Postfix to send emails from external SMTP servers.

Postfix: Sending Emails From External SMTP Servers

If you’re managing a Linux server, you may be wondering how to configure Postfix to send emails from external SMTP servers. Postfix is a popular open-source mail transfer agent (MTA) used to route and deliver email on Linux.

Postfix: Sending Emails From External SMTP Servers – TecAdmin