I presume you are talking about a pop mail account which uses it's own folders - it is not a deferred account using another Global Inbox like Local Folders mail account.

In Folder Pane, in the pop mail account, right click on the 'Deleted/Trash' folder and select 'Properties'. 'general Information' tab Location: ....what does it say ? eg: thunderbird/fmbwzw4d.default/Mail/[email-provider-name]/Trash Please post info on what it says.

How frequently do you compact the Inbox and Junk folders? I'm wondering if your Inbox is corrupted and therefore not functioning correctly. Usually you do not need to clear out/empty an Inbox in order to compact but on this occasion I suggest to do it just in case something is wrong and you do not want to lose good emails.

Suggest you move all wanted good emails from 'Inbox' into other suitable folders for storage. right click on 'Inbox' folder and select 'Compact'.

then if unwanted emails remain in Inbox, try to delete them.

note the 'delete' button does have an icon which is a circle with a diagonal line. Usually it is grey, but you may be using a theme which offers a colour.

I think you've found a discrepancy. It is (or should be - how do I tell?) a pop mail account, because the transfer protocol for the mailbox concerned is POP3 with SSL/TLS. The properties for the Deleted folder are:

mailbox:///home/[user]/.thunderbird/fmbwzw4d.default/Mail/Local Folders/Trash/

whereas the Profile Directory is:

/home/[user]/.thunderbird/fmbwzw4d.default within which is Mail, within which are "Local Folders" and [email-provider-name]

As for how frequently the inbox and junk folders are compacted: this computer and hence the installation are only a couple of weeks old, so "never yet". I don't think there is a Junk folder.

I bravely tried a "Compact" operation on the inbox without copying the mail elsewhere (we can still access the messages via webmail - although this is POP3, this provider's POP3 implementation doesn't actually delete messages with the DELE command). There was no indication of success or of failure.

I was able to move an unwanted message into a newly created folder called Storage, but I couldn't delete it from there.

Thanks for your note about the icon next to Delete.

I think I've fixed it. The problem appears to be that the installation creates a Trash folder, whereas what it needs is a Trash file. I deleted the folder and the corresponding Trash.msf file and restarted TB, whereupon I can delete stuff.

It doesn't help that the common remedy on the internet says to delete the Trash folder and Trash.msf file, and, for a POP mailbox, recreate a Trash folder. The remedy fixes and immediately recreates the problem.

It would seem you were not following the help info correctly.

re :remedy on the internet says to delete the Trash folder and Trash.msf file, I've located the help info and it says: Step 5. Select the Trash and Trash.msf files and delete them. This is refering to the files in the pop mail account profile folder.

re :and, for a POP mailbox, recreate a Trash folder. The info does not say to create a Trash folder in the profile folder, it says to restart Thunderbird and add a new folder called Trash via the mail account in Folder Pane. Step 6. Restart Thunderbird. If you are using POP: In the folder list, right-click on your account name and select New Folder.... Enter Trash as folder's name and confirm by clicking on Create Folder.

Helpful information on this topic here: https://support.mozilla.org/en-US/kb/cannot-delete-messages