There are several ways to determine if a variable contains a valid email address. These techniques do not check if email address exists, but they determine if the string contained in a variable complies with the format of an email address.
Using a filter
Using a function of the family of filters, you can check that email address is valid.
//e-mail is good
With regular expressions
It is possible to check if an email address, for example via entry form, is valid.
Here is a function that checks if a string is only a valid e-mail address.