def fun(s):
    if '@' not in s or s[0] == '@':
        return False
    else:
        name, rest = s.split('@', 1)
    if not all(c.isdigit() or c.isalpha() or c in ['-', '_'] for c in name):
        return False
    if '.' not in rest:
        return False
    web, ext = rest.split('.', 1)
    if not all(c.isdigit() or c.isalpha() for c in web):
        return False
    if not all(c.isalpha() for c in ext) or len(ext) > 3:
        return False
    
    return True