Try the following on each account. Empty the Trash folder - right click on Trash and select 'Empty Trash. Create some folders Move all good wanted emails out of 'Inbox' into other folders for storage. When 'Inbox' only contains emails you do not mind losing or want to delete: Right click on 'Inbox' and select 'Compact'

Now try deleting any remaining emails from that 'Inbox'. then Right click on 'Inbox' and select 'Compact'. you should now have a clean 'Inbox'

Report back on results.

I suppose that using this method I can get rid off all the unwanted messages. But this does not really solve my problem. What would happen the next time I would want to delete a message. What you suggested is a very cumbersome procedure, and can't be applied in a daily usage. I wonder why my copy of Thundebird does not let me go through the very elementary operation: choose the message you want to delete and press on the delete icon. I tried to reinstall the program, but it didn't work So, what should I do ?

Is your account IMAP or POP?

As I have already told you it is a cumbersome method, and it doesn't really solve my problem. But I managed to find a solution. I removed the problematic account from Thunderbird, and added it once more, defining its parameters from the start. I am happy to report that it did the trick Adam Rom