Emails are stored in the mbox files - which is a text file with emails appended one after the other in the order downloaded.

Thunderbird uses an index file to display the list of emails and it calls on the mbox file to display contents of actual emails, but if the index file gets corrupted/out of synch with what is actually in the mbox file, then it needs a repair.

Advise: Do not allow your folders to get too large because it's putting all your eggs in one basket and is more risky if something goes wrong. Also, if you need to access those mbox files, it can be difficult to open the file if they are in excess of 2 GB and I would say ideally not larger than 500MB. I don't let them get larger than 100MB. Extremely large text files can use enormous amounts of RAM if you try to open them and can cause computer to crash. Thus I advise you avoid that potential senario.

Create more folders to organise emails for storage eg: Family, Friends, Bills, Online Orders, Thunderbird, etc etc.

In Account Settings > Copies & Folders set up 'Archiving Options' and archive anything over a year into yearly folders. You can also select to maintain folder structure. Archiving can help to keep folders more manageable and reduce risk of loss. After Archiving emails from a folder - compact the folder.

Compacting: Compacting is not compressing. Compacting the folder removes any downloaded emails which you have since moved or deleted, thus keeping the file tidy. Compacting is very important especially on Inbox, Drafts and Junk/Spam folders which get more moving and deleting than other folders.

Basically -The mbox file gets copied and Thunderbird scans through it looking for good wanted emails which it puts in a new file. Those emails that have been 'marked as deleted' are not copied. Then new file replaces old file and new index is created. File size is reduced and made tidy. Compacting in itself does not corrupt the file, but if file has not been maintained then it is more likely to be already corrupted and compacting it may end up removing more emails than you desired. Hence my advise is to create folders and organise emails into smaller files.

If you are seriously worried about eg: 'Inbox' You need to completely empty the 'Inbox'. Create folders and move emails into those folders. Then archive all the old mail. When Inbox is empty - right click on Inbox and select 'Compact' This will in effect create a nice new Inbox file completely empty and ready to receive new emails.

Thanks Toad Hall. (I think I should say "Ratty here" but I think I'm more Mole.) This is helpful and clarifies some things. You say: "Thunderbird uses an index file to display the list of emails and it calls on the mbox file to display contents of actual emails, but if the index file gets corrupted/out of synch with what is actually in the mbox file, then it needs a repair. " So I take it that when the display of the Email was corrupted what was happening was that the index, presumably in a relational database, no longer pointed to the correct place in the mbox (text) file and hence what TB showed me was a mess. That makes sense if the index was pointing with an offset so much of the formatting information at the top of the body of the Email was missed. I think that fits perfectly with what I was seeing. I guess to me this raises a huge question about why and how the index is getting out of synch and perhaps that is related to the compacting and the index not getting rewritten to reflect changes the compacting had made. Question: is TB looking at the mbox file on the IMAP server or copies on the local hard disc? Where would I look to see the mbox folders and their sizes? (Local is Ubuntu, IMAP is on a shared access Debian machine run by a very experienced and competent ISP. I can follow this up with them when I understand it better.)

I understand the issues about sizes of folders. I actually have many years of Emails stored in very many folders in a hierarchical structure. The inbox is not the biggest folder and I think so far I've seen one corrupt Email there but a couple in other folders. I _was_ allowing TB to compact whenever it wanted (and it now wants to do so very often). Again, I guess pretty much repeating the last question, is compacting local or on the distal IMAP server, or both and having to keep them in synch? I have quite slow broadband here and with some large folders, and many folders, I could understand if some synching between the local and distal stores is getting broken by time outs in the link.

Is there somewhere you recommend for me to read more about distal and local storage and about how archiving fits into that? So far I haven't used archiving as I was reluctant to add more complexity to the storage systems but you are making it sound like a potential solution to some of the (putative) issues.

Thanks again and TIA for a bit more information/pointers. Chris/Mole/Ratty

re :Question: is TB looking at the mbox file on the IMAP server or copies on the local hard disc?

Depends upon whether you have downloaded full copies (fully synchronised) or only headers. Please note that for imap accounts, you can set this up on a per folder basis.

Some people only download headers as it saves a load of space on their computer, so when you select to read an email, the index file is used to identify your selection and Thunderbird has to jibber jabber with the server to locate and download a copy which it stores in a temp cache to facilate viewing.

See info on IMAP: https://support.mozilla.org/en-US/kb/imap-synchronization

re :Where would I look to see the mbox folders and their sizes?

OK so you are using a Linux based system Ubuntu. Thunderbird profiles in Ubuntu are stored in the (hidden) '.thunderbird' folder in your 'home' directory.

• ~/.mozilla-thunderbird/<Profile name>

You can locate the info via Thunderbird.

• Menu icon > Help > More TRoubleshooting Information
• Under 'Application Basics' - half way - Profile folders - click on 'Open directory'

A new window opens showing the contents of your profile name folder.

• As you use an IMAP account - click on 'ImapMail' folder
• click on the imap account name folder

Each folder in the folder pane will have some files of same name.

mbox files do not have any extension eg: 'Inbox' and they contain downloaded emails. Emails are written to file one after the other in the order downloaded, so oldest at top.

index files have same same as folder but they have a .msf extension eg: 'Inbox.msf'. If you do not download full copies then you will only see .msf files.

Subfolders - if you have any subfolders then you will see folders with a .sbd extension. So if your Inbox folder has subfolders then you would see 'Inbox.sbd' folder. Those folders may contain mbox and .msf files or even .sbd folders.

Compacting is performed local on your computer.

Please note if you need to perform a backup of the '.thunderbird' folder or the 'profile name' folder itself then remember to exit Thunderbird first.

Thanks for both messages Toad Hall. I'm 99% sure I've always shut TB before backing up the profile (actually, my system does that nightly with a cron job copying the profile to me fallback Ubuntu machine). The other information was spot on: I rechecked and I am copying everything down on the local machine and not using the automatic deleting on age. That's best for me as I have a lot of disc space but slow internet so don't want too much "jibber jabber" (love the term) ... and it reassures me that I have local copies. I think it's clear now that the corrupted messages were happening when presumably something messed up the indexing and that the uncorrupted messages remain available locally in the mbox file (and on the IMAP server) and that I can overcome the corrupt display be using the "Repair folder" re-indexing in the properties for the folder so I have marked this fixed. Hugely appreciate your help.