If you're copying from text in a wordprocessor, try using the paste unformatted menu option.

Dear David,

Thank you for your reply. I did as you said, but the text was still jumbled when it was transfered to the e-letter I wanted to send. I ought to mention that this problem began after I attempted to enlarge texts so that I could better see them, due to difficulties with my eyes. I have gone through the enlargement procedures, but see nothing that may be a problem, but I may be missing something. As you may remember ,this may not be the first time.

Cheers, James S.

It may help if you give explicit detail on the steps you take from wordprocessor to Thunderbird. Can you do a screenshot of the jumbled text?