Hello, did you always keep all your incoming messages in the same folder Inbox or did you move or archive messages to other folders? Is this an Inbox folder in a POP or an IMAP account? Which is the size of the Inbox folder and how many messages does it contain? R-click on Inbox > Properties > number of messages & size on disk.... Do you have a backup copy of your Thunderbird profile folder? Did you change your retention policy last year? Account Settings > account name > Synchronisation and Storage > Disk Space...

Hi and many thanks for replying. I kept/ stored most emails in inbox (just a few in other folders). Did like this in the past 8 plus years and had never problems. I also got no advice if there is a certain limit to keep mails in inbox. Is this the case?

It's IMAP.

I don't have an exakt number how many mails I had in my inbox, but it was a good amount of the past 8 years for sure. For example on my other computer I have emails from around 15 years in the origin Apple Mail application.

Right now it contains 4587 messages and takes 452 MB. I guess it had way more before the lost.

I don't know if Thunderbird did backups automatically. Maybe my fault I didn't consider doing it manually.

I didn't change anything in the past besides when those prompts appeared to compress datas.

There is no space limit for storing. Inbox offline reading was/ is also activated for offline reading. 'Deleting messages when in need for space' was on 'no messages delete'.

You certainly didn't change the default mailbox (also called mbox) storage format, which means that all your messages have been stored in one text file names Inbox. And unfortunately an Inbox files gets bigger and bigger if downloaded messages are not regularly archived or moved to other folders, and it might get corrupt. Thunderbird doesn't make automatic profile backups, it's up to the user to save his data.

Nevertheless verify the size of the INBOX file in Thunderbird's profile folder: go to Help > Troubleshooting Information > Profile Folder > Open Folder (if you are on Mac: Show in Finder). In the profile folder open the ImapMail folder containing then an "imap.xxx.xx" account folder and look up the size of the INBOX file (the one without suffixe). Also ask your email address provider if the have a backup of your account.

Thanks again. Ja, I didn't change anything, not in box, not somewhere else. I'm pretty sure something got corrupt during the last compression process. I wasn't aware at all that I have to drop mails from inbox into another folder! Maybe this point should be highlighted more. I had a look for that inbox file without suffix. It's about 503 MB. Sadly nothing more than the size of emails in inbox. No chance. They are gone. But they are still on my iPhone. So I have to find away to copy them from phone and transfer them to computer and then into Apple Mail App.....

But they are still on my iPhone. So I have to find away to copy them from phone and transfer them to computer and then into Apple Mail App.....

If you have set up the your email account in Apple Mail on your iPhone you can set up the same account as an IMAP account in Apple Mail.app on the desktop Mac. This way Apple Mail on iPhone and on desktop will be synchronised.