Not understanding why you want authentication for sending emails. SMTP authentication doesn't means authenticate before sending each mails. It is using for protect your server from being an Open Relay. SASL authentication normally working on port 465 for users who want to relay mail to other domains.
If you enable authentication on port 25 , not any other servers (domains) will be able to send mails to your Server. bcz it wont allow to connect on port 25 without authentication.