In Sharepoint we need to have the email validation in the list forms under some circumstance to complete the task,Email validation helps the user in not entering wrong email .In other way
Email validation means to check an email address against certain rules to verify its existence. Generally email address validation is done through Format Check, Mail Host Check and Username Check. Note that although the SMTP check can determine whether an email address is bad or not, it cannot guarantee an address is valid as not all mail servers provide notification of invalid user names.
Check out this video on email valition to know how to perform validation in
