In latest versions of Mac OS X, by default the postfix is set to send emails from your local machine to outside world. This might cause you troubles if you are developing applications on your local machine and your application has email sending functionality to customers. Unfortunately if you have some customers’ email addresses in the database or in your code in the local setup, then test emails get delivered to them which is embarrassing. This happens even if you have not setup/integrated any 3rd party smtp services in your application.
To avoid this kind of situation, you can stop outgoing emails being sent through postfix from your local machine. Simply put the line below into the `/private/etc/postfix/main.cf` file in your Mac OS X.
`default_transport = error:No outside emails.`
The variable `default_transport` specifies which transport is used to deliver non-local mail (default is smtp). With this setting, any outgoing mail will bounce back with the error specified.