Which option in the Postfix configuration makes Postfix pass email to external destinations to another...

The relayhost parameter in the Postfix configuration file (/etc/postfix/main.cf) specifies the hostname or IP address of an external SMTP server that will handle the outgoing emails for Postfix. This server is also known as a smarthost or a relayhost. By using a relayhost, Postfix can delegate the responsibility of delivering emails to remote destinations to another server that may have better network connectivity, security, or reputation. A relayhost can also provide authentication, encryption, or filtering services for Postfix. References:

Postfix Standard Configuration Examples

How to configure Postfix relayhost (smarthost) to send eMail using an external smptd