Here is another example, even more garbled than the last one.

I deleted the attachments because I didn't want email info to possibly be seen by just anyone, so here are the edited screenshots.

They don't show anyone's names etc, but they show the issue.

Thanks, Russ

And here is the other one...

The symptoms you're describing sound like your mail files got corrupted. The most likely culprit for corruption is anti-virus software. That means data loss may have occurred already.

Try to rebuild the index file of the troubled folder, which should fix the corruption.

Right-click the folder - Properties - Repair Folder

Note that the repair cannot and will not restore messages already lost. If that is the case you'd need to restore them from a backup you created prior to the corruption.

These are some generic suggestions to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Thanks for the reply, and I can see that causing an issue, but I am confused on a couple of points.

1. 1 Would this only affect the messages within Thunderbird or corrupt the whole message? The reason I ask that the messages in question are actually intact on the Yahoo server. I can login to their website directly and view the original messages without the html.

So would re-building essentially grab the messages from the Yahoo server and re-import them into the Thunderbird folders? If so this would likely be the right solution.

1. 2 Would an issue like this affect the entire folder, or could it still just hit individual messages. If it affects either entire folders or all new messages into a folder, then that isn't what I am seeing. It is just a few random messages here any there.

I do use IMAP and there seems to be some issue with Yahoo and IMAP as I get occasional error messages with password verification etc. (I blame Yahoo for this), but I am not sure if there may be some other stability glitches between the two platforms?

Thanks again for the help and look forward to hearing additional feedback.

So would re-building essentially grab the messages from the Yahoo server and re-import them into the Thunderbird folders?

No, It's actually the opposite. The damage has been done already. The repair attempts to fix that. As a result messages may then disappear locally and also from the server. Therefore it is recommended to create backups on a regular basis.

Would an issue like this affect the entire folder, or could it still just hit individual messages.

I guess it could be both, depending on how severe the corruption actually is. In any case, Inbox is most susceptible to corruption, as the file is written quite often when new messages arrive, or are moved to another folder.

I do use IMAP and there seems to be some issue with Yahoo and IMAP as I get occasional error messages with password verification etc. (I blame Yahoo for this), but I am not sure if there may be some other stability glitches between the two platforms?

The exact error message may help. In general, Yahoo isn't the most stable and reliable email provider, but overall it does the job.