SMTP authentication is a function to authenticate a mail server when sending an e-mail.
There are three kinds of the authentication systems for SMTP (Simple Mail Transfer Protocol) in the following.
1. No authentication
2. SMTP
The system which is authenticated by ID and password of
an e-mail account when sending an e-mail
3. POP before SMTP
The system which is authenticated by connecting POP server
before sending an e-mail
*This avoids taking advantage of spam mails