I strongly advise that you back up your Thunderbird profile before you try this solution! If you aren't sure how to do that then please ask here before you proceed. Once your profile is safely backed up, go to Preferences > Advanced > Config Editor Then enter 'wrap' (without the quotes) in the search bar. If the email you are viewing is in plain text rather than html you may need to change plain_text.wrap_long_lines to 'true'. You will need to close the editing window and maybe restart Thunderbird before the change takes effect. If that doesn't work you may need to change one of the other parameters, I can't tell from the description in your message. Experiment -- but at your own risk, OK?

Thank you for your kind reply. Unfortunately, this is way beyond my ability with a computer, I believe, but I may try it at a later date. Now I have a worse problem. I tried forwarding a rather large photo that had been sent to me. Since I BCCed it to myself, I filled up my web mailbox and was told to delete my old mail (mail that had already been downloaded to my computer) and purge the trash to make more room. When I did that, all of the email in my Thunderbird Inbox and Trash disappeared also! I do back up with a Western Digital external backup device, but I could not find the email. I tried to restore everything that said Thunderbird, but I didn't get my email. I didn't try restoring something labeled Mozilla, so maybe that was the problem. Now I'm not sure if my WD is really backing up anything, and I've lost some important email. Any thoughts? Thanks again.

Issue 1. You are apparently using IMAP to manager you mail. I suggest you tread a little about how it works. Then normal actions like your seeing will not be issues. We have a summary here https://support.mozilla.org/en-US/kb/glossary-terms-including-types-accounts#w_imap

Issue 2. With your new mail, press Ctrl+R. That should cause the mail to wrap correctly as it issues a rewrap command.

Thank you, Matt!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Yes, I do use IMAP. The article you mentioned certainly did help me understand what is going on. I may switch back to POP, which is also offered by my ISP. I wasn't able to use your second suggestion regarding the Ctrl+R because the problem fixed itself! When I tried replicating the posts that were doing this, I couldn't recreate the problem. However, I will definitely keep your suggestion on hand in case the problem occurs again. Thank you, Matt, for taking time to answer my questions and for being a moderator for Thunderbird. Sherry in Oregon

Oh, I read in a Thunderbird instruction page that once an account using IMAP is set up, there is no way to go back to POP. Is this still true? One article mentioned starting a new account using POP and cancelling/deleting the old account. Will incoming email be lost then?