Can anyone advise: what I should do about this recurring message: iTunes can’t verify the identity of the server “sp.itunes.apple.com”.

I am running OS X Yosemite 10.10.5 on a MacBook Air .

Can anyone advise: what I should do about this recurring message: iTunes can’t verify the identity of the server “sp.itunes.apple.com”. I then have to select either Continue or Cancel so I always chose Continue and it seems to work. But why can't it verify the ID?

Thanks davidfromhythe