Try as I may, I can not find this setting referenced in release notes - can anyone help me locate it? Has it made a difference to anyone?
Prevent Incoming Spam with Secure Aliases
Email aliases are great for saving time when sending messages to multiple users that you communicate with often. However, if a spammer gets ahold of your company aliases, it could mean an increase in spam. SmarterMail 13.x adds alias protection by including an option to only allow emails to be sent from addresses that match the alias domain.