Try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder

Thanks. I'll give it a try and see what happens the next few days.

Since at least two updates ago (currently on 68.3.1) I find a small number of emails are blended together in the following ways:

• Two emails are concatenated with only the first subject line showing. HTML correctly displays for the first email. The second is not displayed. I discovered this when a succeeding email displayed the contents of the previous one. Thus began my investigation as problems became more complicated.
• Two emails are concatenated with only text rendering, not HTML rendering shown.
• Two chained emails are are indicated with the first one's subject line but no body shown or rendered except perhaps for a background banner.
• No text or rendering is shown but saving the email shows text is present when viewed in a text editor (Notepad++).

Repairing the Inbox folder does not fix the problem. Some emails even disappear.

It is still happening for me but not very often. I'll try that notepad trick.

Most likely your anti-virus software is corrupting your mail files.

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. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

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

Isn't it risky to not scan incoming mail and attachments for viruses? I backup my system regularly which included includes Thunderbird. I use McAfee for A/V.

I use McAfee for A/V.

See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#McAfee

Thanks!! I went to the link. I am going out to dinner with some friends now so I will give that stuff a try tomorrow.