def is_email(email):
    """
    Simple email validator.

    A valid email contains only one @ and the @ exists neither
    at the start nor the end of the string
    """
    # email length - 1 for indexing into the address
    email_len = len(email) - 1
    count = 0

    # At least 3 characters are required for a valid email
    if email_len < 3:
        return None

    # Short circuit on invalid findings
    for i, v in enumerate(email):
        if v == '@':
            count += 1
            if i in [0, email_len]:
                return None
            elif count > 1:
                return None

    # Upon searching the entire "email address" and the loop wasn't
    # short circuited, a valid email was passed
    return email