multiple emails all going through the same profile path

Please explain your setup in more detail (Unix user accounts, Thunderbird profile(s), profile location, email accounts).

We have 3 Linux accounts 1 Administrator and 2 User accounts. The administrator account does not use the same Thunderbird profile as the 2 user accounts as no-one accesses email through it. The 2 User accounts use the same Thunderbird profile directory which is stored on a separate hard disc to the Ubuntu installation. This is where all our data is held. The 2 Ubuntu user home directories are also held on this disc but are separate to the Thunderbird profile directory. The Linux Administrator home directory is held on the Ubuntu installation disc. We have 3 email accounts, all through virgin media. All the account settings (Ports, Server settings etc.) are the same when checked through either Ubuntu user account. I did think it could be a permissions issue but looking at the files within each Thunderbird email account directory doesn't shed any light on the issue.

I think I may have found the issue, but not the solution yet. When Thunderbird is started from my wifes Ubuntu login Some of the mailbox files are being altered to unknown file types. If I subsequently view them in Thunderbird from my Ubuntu login, the file type alters back to a mailbox file. Further investigation of the settings in my wifes login is probably where I'll start.

I did think it could be a permissions issue

Assuming you use a Unix file system on the separate disk. The two user accounts need to belong to the same group. All files in the profile need to be owned by that group. The group needs to have 'write' permissions. You probably know that, but just in case.

I'd certainly use separate Thunderbird profiles for each user.

Some files are being altered to unknown file types.

I'm not sure what this means. But I'm not an Ubuntu user.

The original files were set up as an earlier user I had created in Ubuntu. I had originally given read/write permissions to all users so that everyone could access them. I started by using separate Thunderbird profiles but my wife was relying on me to tell her when she got an email, so it became easier to have just the single profile so that emails weren't being spread across several profiles.

The unknown file types seem to be altered in some way so that they aren't being recognised as mailbox files.

I have also now discovered that the file permissions are being altered so that the person who opens the directory in Thunderbird gains ownership of the file and blocks all others out. I'm now wondering if this could be a bug, as everything was working OK until recently.

Update:

The files that had the permissions altered were the ones causing the problems. Ubuntu must use the file contents to determine them to be mailbox files. So if another user can't access the file contents Thunderbird is unable to display the messages held in that file. I altered the permissions of all the files with corrupted permissions so everyone can access them and that has solved the problem. The only question now is how did the ownership and permissions of these files change in the first place?