It depends if the emails are held on a server or downloaded to your phone/laptop.
If they are held on a server there is only one copy of the email and if you delete it from your phone it deletes it from the server and wont appear on your laptop.
However if the email is downloaded from the server to your phone AND your laptop there is now two copies of the email and if you delete one (say off your phone) the other is still on your laptop and has to be deleted separately.