I have finally found out why I could not retrieve my mail through Thunderbird, (authentication always failed.)
I thought I would post the solution just in case anyone else falls into the same trap as I did.
My password included a £ sign and this character causes authentication to fail, when I changed it to a - (dash), everything worked fine. I do not know if there are other characters that do not work but if anyone has this problem it could be worth trying different ones.
Linux always encourages the use of strong passwords but it seems like you can make them too strong!
I gave up with SME Server at the time and used Zentyal but I'm not one to give up easily and now I'm back!