Good news, I changed to Mozilla, sent and received messages well with P.C's and Macs. Thanks for your help.
Whilst to me Mozilla is not as easy to find all the functions as K-mail, it works.
An interesting aside to the installation was on a Mozilla help site where there was an article on how to reduce spam (with which I am inundated) which recommended in order for the spammer not to realize that his message was received/read, is not to bounce or delete the spam on line, as there is a signal returned so that they know your addresss is a live target, after legitimate mail has been read log off, then delete the spam. I had always wondered how to delete spam without opening the message.
Now I have printer problems but that is for another post.