I HAVE ATTACHED AN EXAMPLE

There are a couple of preferences that can be changed that might fix this. Open Tools/Options/Advanced/General/Config. editor and double-click a preference to toggle its value.

mail.imap.mime_parts_on_demand change to false

mail.server.default.fetch_by_chunks change to false

Change one at a time and see if the attachments open without error. Try it with a new message to yourself. View the message source (Ctrl+U) to force the download of existing attachments.

I am working with Thunderbird. I cannot find a 'Tools' tab. All I find are 'get messages' 'write' 'chat' 'address book'. I am obviously not conversant with much beyond that. Where do I find the 'Tools' tab?

Press Alt if the Menu Bar with Tools is hidden. Or, click the 3-bar icon, Options/Options/Advanced/General/Config. editor.

Tried both suggestions, problem persists..

What's the security software? Issues with attachments are often caused by antivirus apps, especially if they are set to scan mail folders. Add the TB profile folder as an exception or exclusion in the AV settings and see if that fixes the problem. Also, check if the attachments are broken when the account is accessed through webmail, to test if the corruption is present before downloading to TB.

I am running Kaspersky security. I have checked settings and find no way to except TB and have no idea how to test corruption before down loading.

Add the TB profile folder as an exclusion in Kaspersky. Help/Troubleshooting, click about:profiles to see the profile folder location.

Access your account through the mail provider's website and see if you can open an attachment without error. If it opens properly, that suggests the problem occurs after the message is downloaded to TB. If it doesn't, the attachment was broken before it reached TB.

Figured out a work around... Save attachment to where-ever right click on file>properties>unblock... then opens fine. Now I just need to figure out where it is getting blocked.

There are a lot of discussions on how to avoid 'blocked' files, e.g. open Word or Excel, Options/Trust Center and uncheck all options for Protected View.

Thank you for your patient guidance sfhowes