def fun(s):

    try:
        username, rest = s.split('@')
        websitename, extension = rest.split('.')
        
    except ValueError:
        return False
    
    if not username.replace('-', '').replace('_', '').isalnum():
        return False
        
    if not websitename.isalnum():
        return False
        
    if not extension.isalpha() or not (1 <= len(extension) <= 3):
        return False

    return True