message content may be completely blank, or be the content of an another email adjacent in the list.

It sounds like your mail files are corrupted.

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

Note: depending on the corruption this may erase messages from the affected folder which cannot be recovered anymore. In that case you'd need to restore them from a recent backup. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

Corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

For more information on the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Inbox is most susceptible to corruption. It is therefore best practice not to accumulate messages in Inbox. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

This appear to have started around the time that Norton detected and quarantined a couple of emails with identified virus attached.

This is exactly the reason not to let anti-virus software mess with your Thunderbird profile. Any malware possibly attached to an email will not do any harm as long as you don't open it and deliberately open or run the attachment. Let junk mail control take care of spam messages. http://kb.mozillazine.org/Junk_Mail_Controls

Don't open messages with attachments from unknown sources. Simply delete them.

Don't open attachments you haven't asked for.

Thank you for your explanation it explains & confirms what I had deduced from the events. Having fallen back to the previous nights backup I was able to restore mail files will the loss of only 2 or 3 messages. Unfortunately it doesn't seem sensible to turn off anti-virus scanning because, in business, although you are expecting an attachment and you know the source, the technical capabilities of the source are not always known. I take daily, or if busy, twice daily backups, so I think I'd prefer to run the (now known) risk of losing a few emails rather than go through the pain of trying to remove something nasty that came from a known source.

Some form of holding directory that would permit scanning prior to arrival in the mail application would be nice, but not I suspect, practical. (my mail service already stops many suspect & spam messages but can't stop all problems particularly within attachments).

Thanks

Unfortunately it doesn't seem sensible to turn off anti-virus scanning because, in business, although you are expecting an attachment and you know the source, the technical capabilities of the source are not always known.

I didn't suggest to turn off anti-virus scanning. That would indeed be a bad choice. The point is not to scan attachments while they are still part of the email message.

Some form of holding directory that would permit scanning prior to arrival in the mail application would be nice

I don't know what you'd want to achieve by doing that. Again, any malware possibly attached to an email will not do any harm as long as you don't deliberately open or run the attachment.

Saving an attachment to disc and then let it scan by anti-virus would be a good strategy.

1) that would require detaching & scanning every attachment (because until they're scanned you don't know if they're infected of not) before opening or running - quite an overhead.

2) using a holding directory or front-end scanning routine prior to passing the mail item to the email application has the same objective, ie to perform the anti-virus scan outside the mail application file system.

I don't know the amount of attachments you do have to process. I suppose there are Enterprise solutions available which scan messages and attachments directly on the server, before a message hits a user's mail box. Most email providers also do this for private user accounts. If in doubt ask your email provider.