How many types of email authentication?

Typically, an email server will allow you to send out messages by verified email account. (authorisation) If you can send messages without authentication of user that means anyone can send messages to others, which is not safe surely. And the identity of the email hosting services are as follows.

How many types of email authentication?

Using Username and Password 

This method is the most popular and widely used because it can be set in the end user email client easily. Mostly, the user just authen by username and password which are the same as Incoming Server.

Whitelist IP 

This way, the email hosting services provider allow a client who is a large organisation and ensure that the network is secure enough to permit any computer's users sending email messages out without any verification. Simply being in the actual IP address of the admin you have provided to the provider.

Email Header 

This method is highly technical. The email hosting services provider will require code to the clients for putting them in the email header, when a message is sent to the email server, it will scan the email header matched, so the email messages could be sent out.