It look like email sent from a Microsoft application, perhaps Outlook, with different encoding (RTF?). Sometimes this includes winmail.dat attachments. Whoever is sending you these emails would need to change their formatting to HTML or plain text.

Check if installing this extension helps, you should see file attachments that you can then open:

https://addons.mozilla.org/en-US/thunderbird/addon/lookout-fix-version/

Thanks. You are correct that the emails I am receiving are from users using Outlook. I installed the extension, but that did not help.

I talked to one person who is sending the emails that I can't read. He said that he recently changed his encoding from UTF-8 to UTF-7 to solve another problem he was having. In my Thunderbird, I have View->Text Encoding set to Unicode. Any other ideas?

have you tried right clicking the folder, selecting properties and then the repair button. Most instances of unreadable emails are related to the index used to locate them in the mail store. repair rebuilds the index.

If you have this problem often, you might want to look at not allowing your anti virus to scan in the Thnderbird profile folder when the programs is running.

Email received from a sender using Outlook includes a Winmail.dat attachment, should be in decline as Microsoft list the following applications as applying to.

Applies to: Microsoft Office Outlook 2007Microsoft Office Outlook 2003Microsoft Outlook 2002 Standard Edition, Microsoft Business Productivity Online https://support.microsoft.com/en-au/help/278061/email-received-from-a-sender-using-outlook-includes-a-winmail-dat-atta

Thanks, but repairing the folder did not help. There is no winmail.dat attachment. Unfortunately, my workplace is trying to get people to move to Outlook, so if there is no fix for Thunderbird, that's a problem. Any idea why Mail on my iPad and iPhone can read the messages?

Thanks for your help!

What do you have in your Fonts & Encoding Preferences ?

Ideally both incoming & outgoing Text encoding should be set to UTF-8.

Thanks, but I already have both set to UTF-8.