Here's another option. At least in the new designer, the field with the formula will show a formula error if the source email field is blank, or if doesn't contain the @ symbol. Based on your needs, that error may be acceptable - but if that is a problem, then this formula should be able to get the same result while avoiding the error:
=INDEX(SPLIT(EmailField,"@"), 1)
The inner SPLIT formula breaks up a string into an array, split each time it encounters the indicated character, so email@website.com becomes an array of email and website.com. Then the outer INDEX formula selects the first item in that array of values.
Unlike the FIND formula that will generate an error if the searched character isn't found, the SPLIT formula returns a value even if the character is never found.
{BLANK} will evaluate as {BLANK}
email will evaluate as email
email@website.com will evalutate as email
email@website@com will evaluate as email