After further investigation, I've found that I couldn't send email on either Yahoo account, although my wife can without problem on her Yahoo accounts. Experimenting further, I then found that I could send email if I saved the email as a draft and then went to "mail" again, then "draft" , selected the email message that I had just written when I was in the "compose" mode, and then press "send".

Experimenting even further, I found that I could send email if I did not select the address from the address book but wrote it in manually! This is weird. I thought that perhaps someone put a virus in my computer to do this but I've run several virus/ adware/spyware scans and my computer is clean. I'm perplexed.

Richard