Question Author
Thank you all for your helpful comments.
I must apologise for the delay in posting this reply but I have been out of circulation for a while.
Hans: Thanks for your MWB suggestion. I use this on a regular basis but it had no effect this time. (Nice try though!)
Also I take your point about your "Thanks in advance" comment but it had never been my intention not to reply to all ABers who take the time and trouble to reply to my query.
Unfortunately, the Email is still apparently stuck in my Outbox and won't go away so I'm baffled.
The good news is that I only used my Yahoo email for a very limited number of recipients so I have been able to close it down without too much problem. Also I have other email accounts (including, as Buenchico will be pleased to hear, GMX), each with a dedicated purpose - so life continues.
The cause of my original problem remains a mystery though but I can only reiterate my thanks to all who offered their help.