After carrying out some research through Google it became apparent that the problem was not a Thunderbird one, but the mail server, in my case hosted through GoDaddy. Several individuals had reported through various forums that they had solved the problem by logging into the mail server and clearing out junk files. In my case I found about 400 files in my inbox but the other boxes were empty. Anyway, I deleted all but one message, which would not delete. I tried moving the message to the trash box - it would not move. Naturally I assumed this file to be the cause of the problem - not so.
On logging out of the server I sent a test file to myself and tried to download it - to no avail, and when I tried to log back into the mail server again I got this message:
Clearly this was going to be a case of summoning the technicians at GoDaddy, and I had Liz (now in New Hampshire while I am stuck back here in South Africa trying to sell the farm) contact the GoDaddy team to attempt to resolve the problem - and they did. They had Liz log into the mail server from the back end, and here's what she found in my inbox:
Yes, you saw it right - more than 4 billion files in my inbox ! 4,294,967,295 files to be precise! Well to cut a long story short, they deleted the files - but I guess down in Phoenix they're still talking about how they got in there in the first place. My immediate thought was in the direction of a virus or similar. I would appreciate comments in this regard.
Anyway, the problem has now been solved and I am once more able to download my email messages and to log on to the mail server if needs be.
Please contact me for Internet content of any kind - you will not find better!