I have more information about my problem:

As I wrote, the corrupted message had the contents of the first message in the IMAP's Inbox but the "header stuff" seemed to be ok.

When checking the "corrupted" message on my phone with its native e-mail client I noticed that it looked ok. Pfew! No message seems to be lost beyond recovery!

But still, this observation seems to confine the issue even more to be in Thunderbird and not in e-mail accounts/providers.

BR, John

It sounds like your mail files are corrupted. Are you accumulating messages in Inbox? If so, this isn't best practice. Always keep your Inbox as tidy as possible.

First of all create a full backup of your Thunderbird profile. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

With an IMAP account particularly make sure you have a local copy of your Inbox messages.

Then 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.

In case of IMAP this would also mean messages get erased on the server.

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

Hi Christ1,

Yes, I am accumulating messages in my IMAP inbox. The reason for this is that I can then access them from my PC and from my mobile devices, also after having downloaded them to my PC. Most messages will be archived by dragging them to a local folder, or will be deleted, but some I simply want to keep on the server for access from other devices.

Thanks for pointing me to the folder repair option. It fixed things and I can now see the original message again.

So, although the effects of the issue are repaired now, there is only the cause to be fixed in Thunderbird. I mean, it looks like some kind of issue with indexing the message's headers/properties and the contents when moving around messages. Since the message looks intact when looking at the server from another device and the existing repair option could fix it, I think the issue is in Thunderbird itself and not the mail server.

Thanks, John

So, although the effects of the issue are repaired now, there is only the cause to be fixed in Thunderbird.

The underlying cause is often anti-virus software, and thus outside of Thunderbird. Follow the instructions wrt anti-virus software as per the previous post. https://support.mozilla.org/en-US/questions/1104644#answer-831755

Hi Christ1,

I understand that anti-virus software can mess up mail folders. I actually had bad experiences with one from Fortinet before so I am not using that anymore.

Meanwhile I configured my current anti-virus software (Windows Defender) to exclude the TB profile folder, rebooted to see that the change really took effect, and repeated the test. Result: The problem still happens. So I don't think that my anti-virus SW is the culprit.

Moreover I think that excluding mail folders from anti-virus scanning is not a good idea as one way that viruses spread is through e-mail. So unless my incoming e-mail is scanned through some other mechanism, I'd rather keep my TB profile in the list of locations to check.

Thank you and best regards, John

So, although the effects of the issue are repaired now, there is only the cause to be fixed in Thunderbird.

I was under the impression that the problem as per your OP has been fixed.

The problem still happens.

What still happens?

Moreover I think that excluding mail folders from anti-virus scanning is not a good idea as one way that viruses spread is through e-mail.

Anti-virus vendors trying hard to convince people scanning email messages is important and increases their security. And many people even believe it.

In reality, as long as a malware attachment is sitting in an email message unopened, nothing at all will happen. Infections happen because people foolishly open spam messages and deliberately open and run attachments instead of using their brain.

Even when running a malware attachment, the on-access scanner should catch it.

If scanning your mail files makes you feel better, just go ahead. But be prepared for problems coming your way (again).

Hi Christ1,

> What still happens?

When archiving a message to a folder that does not exist yet, like the yearly archives, and dragging the message back to the IMAP Inbox: Although the header information appears to be correct, the message's contents is shown as that of the first (oldest?) message in the Inbox. This happens with or without having my anti-virus software scanning my TB profile.

Archiving to a folder that does not exist sounds odd but this is what happens with (accidentally) pressing 'a' or clicking the "Archive" button while having a message selected.

What is solved is that I can restore message this to the original contents by using the folder repair option.

Meanwhile I did some further experimenting: The above contents corruption does not occur when archiving/restoring from/to a local folder. In the original report I already indicated that it also does not occur with POP mail accounts. I also found out that the problem also occurs for other folders than the Inbox on IMAP mail accounts and that sometimes the contents of the message dragged back is set to that of another message than the first one in the folder.

Best regards, John