To all those who have responded for which I am most grateful! I seemed to have solved the problem. I had tried to add a Not junk button to my tool bar but couldn't find one offered. Then I read somewhere that the arrow into junk on the junk button changes to our if something is labelled junk. The problem as I saw it was that although my posts were hi lighted in brown there was no no junk button appearing anywhere either on the post or tool bar. I then realise my yahoogroups posts were diverted to a folder and were not actually in the junk folder. I right clicked one brown email and chose to send it to the junk folder. From there I was offered a not junk button.
Then I opened one of the browned emails. They all have a button to load remote content. I had not been doing so but when I risked it I found the tiniest junk icon appeared on the brown emails. So tiny I could hardly see it. I screenshotted it to see if I could magnify it but actually when magnified it was still impossible to see any arrow. However, by risking clicking on it a not junk button did appear on the email which I was able to use.
It was only when I clicked the banner agreeing to open remote content that this almost invisible junk button appeared!
I worry a lot at my advanced age (70 this year) that people who have started to rely on computers and actually are being forced to rely on computers will actually find the constant changes in OS difficult to cope with as they age especially if in addition their eyesight is not perfect!
I've been using computer since the 80s and in fact at work we had a room sized punch card computer in the 70s! I hope Apple and Microsoft will bear in mind that constant change may be easy for the young but less so for the "mature" user!
Anyway thanks to everyone who responded :-)