Why I am able to send email without SMTP authentication in Linux platform
As you are aware, you may need to turn on the SMTP authentication in your email client in order to send out
the emails from your email client.
But however you may encounter a situation whereby you are able to send out the emails in your email client even without turning on the SMTP authentication.
This is because in our Linux server, there is a predefined function present called as "Antirelayd" in our server WHM whereby when its enabled, it triggers the POP before SMTP function.
What is POP before SMTP
An authentication method for mail servers that will allow a user who has received mail through POP3 or IMAP to send emails for 30 minutes without need to re authenticate through SMTP.
In short, it means that the user is already being authenticated when downloads an email through POP3/ IMAP protocol and does not need to be re authenticated for sending out the emails for the duration of 30 minutes.
But however an error will be prompted if the composed email reached more than 30 minutes where the session ends.
Therefore you are always advised to turn on the SMTP authentication in your email client for enabling the emails
to be sent out without any issue.
How to enable SMTP authentication for Microsoft Outlook
How to enable SMTP authentication for other email clients