ok a quick Google informs me that;

mail.imap.expunge_option is a penelope/ Eudora only option. As that died with V3.1  it is dead and buried.

mail.imap.expunge_after_delete when set to true should invoke a compact process if the option to move deleted messages to trash it selected.

Thunderbird does not support the UID Expunge command If the version of Dovecote is before 2.0 is does not support UIDPLUS See http://kb.mozillazine.org/Deleting_messages_in_IMAP_accounts

Lesson is use entries in the prefs only as a last resort. The user interface is the first place to modify preferences.

Matt, Thanks, but I already had "move to trash" option selected. Should it be working, or not? Do I need to check my Dovecot version? That link you referred to me says that it should perform a Compact, but it seems that it is not doing that. (and again, if I do perform the Compact manually, the message really does get deleted properly - the POP client no longer sees it)

ok I just set up the setting mail.imap.expunge_after_delete = true

logged into my yahoo email account

Deleted messages in Thunderbird. Pressed reload on the yahoo page and the message was gone each time. Not exact science I know. but only a few second would have elapsed.

Progress has been made!

I tried the "Remove item immediately" option, which unsurprisingly, didn't work. However, when I then switched it back to "move to trash", IT WORKED! :)

I then created another account, pointing to an existing IMAP mailbox that had already been used by another client. The first option I chose was "move to trash", and again, it did NOT work - the messages were NOT fully deleted. I then selected the "remove items immediately" option, tried the test again, and it of course failed again. But, yes, when I switched BACK to "move to trash", it WORKED! :)

Now, I notice that when it doesn't work, the messages actually are NOT moved to the trash folder. Moreoever, I notice that when the messages are able to be moved to trash, the trash folder has a proper "trash bin" icon. I think it's the action of choosing "remote items immediately" which seems to flag the trash folder as a real trash folder - is that expected?

Thankyou, anyway.

Btw, just at the moment, I'm using Windows XP SP3 (same Dovecot IMAP server)

trash folders under imap can be problematical, as can junk Basically Thunderbird "creates" a trash folder the first time you use it. The IMAP server almost certainly has one and it is probably in the folder list, but it does not get the special bits that make it "trash" until it is used.

Likewise is the folder name the IMAP server allocates does not have to meet any standard. Some examples are Yahoo. Thunderbird has Junk, they have a "bulk mail" folder. Outlook.com has a deleted folder and Thunderbird has a trash folder. That is why the actual drop down is supplied in the setting. sometimes you just have to pick the right one.

Matt, I'm not sure you have understood my previous reply fully.

I had already selected a trash folder that physically existed in the IMAP account. You say it does not get the "special bits" until it is used. How on earth can it ever be "used", then, if it's not a valid trash folder? It's a deadlock situation.

Do you agree that I have found a bug? The action I had to take was to select "remove item immediate", and then RE-SELECT "move to trash". I did NOT change the trash folder when I did this - it had already been correctly chosen.

I'm back on Win8.1-64 at the moment, but I've just re-tested using a fresh IMAP account. This time, it all worked beautifully, and I didn't even have to select any trash folder - it says "Move to this folder" in the pull-down select box - there is no folder actually selected at all, yet it still moved the deleted message into the Trash folder. (and yes, the Trash folder has the trash-can icon, which means it is a proper/registered trash folder) Will re-test on WinXP.

I had THIS problem: http://superuser.com/questions/278745/thunderbird-creates-a-folder-called-inbox-on-imap

I thought the extra Inbox was just something created by webmail, and not very important. :)

I have made the recommended config change.

As I said, on a fresh account, if I don't touch the "Move it to this folder" folder setting (leaving at the default of "Choose this folder..."), it works.

The extra Inbox seems to get created when I change that setting to what it SHOULD be - I click on the pull-down, navigate to the Inbox/Trash folder, and select it. When I exit out of the settings, voila - the extra Inbox folder is created, and I then have to go into the Config Edit and replace "Inbox/Trash" with "Trash" to fix the problem. After making that config change, the "Move it to this folder" folder value gets reset to "Choose this folder....".  :D :D