The two-form authentication (called 2FA for short) is an additional feature that you can enable for your Elastic Email account's protection at any time on the security screen. There, you will find several options including the one below:
When you click "enable authentication", you will be able to choose one from three variants:
Text messages (SMS)
Text messages (SMS) and emails
Depending on the selected variant, a security code will be sent to your mobile or/and computer device each time you (or someone else) attempt to log into your Elastic Email account using the classic credentials (email, password). Entering the security code will grant access to the account.
(Note! Make sure that your cellular service is within reach and/or your device stays connected to Wifi (or has enabled mobile data) in order to properly receive the security code.)