Mail will not send my outgoing messages, incoming is working

Mail will not send my outgoing messages. Incoming is working fine. My ISP tech support said it was due to an apple upgrade and to contact apple support for a "work around". My mac is over a year old so I was offered no support even though in the last year I have purchased three iphones and a new macbook pro last week for my son. Thanks Apple - way to make me hate to love you!