Why did my TB not do the auto-compact thing?

Question

I'm just wondering...
My ThunderBird install has this default setting of "compress folders if it saves more than 20 MB in total".
And I have the Dutch language translation installed (don't know how to change it to English....) but as with many software translations, it uses some bad terms which confuses people.
I believe in English TB uses the word "compact" but in Dutch it says "comprimeren" which always only refers to compression in the sense of ZIP/RAR/GZIP etcetera. Because of that I have never manually chosen to "comprimeren" because I didn't understand what it meant.
I believe from time to time TB has asked me whether I wanted to do that thing, and I still didn't know what it was supposed to do, so I never chose to.
In the end, today, I made a backup of my entire IMAP remote collection and it turned out to be only a meagre ~500 MB whereas the data for Thunderbird itself (full offline sync) took up 1.2 GB.
And then I finally chose to "compress" and now the backup and the actual Thunderbird folders/files are identical in size, save for the TB index.
And that means I have had about 600 MB of deleted messages still sitting in my INBOX.
There is another option that translated to something like "Clean up your Inbox (expunge) when closing?" and I have no clue what it is supposed to do and there is also no tooltip or anything. Does it refer to the compact action? Does it refer to moving deleted (now invisible) messages to Trash? I have no idea and when I open my folders in another client (Alpine) it actually shows these "deleted" flags on those messages that I have supposedly "deleted" from Inbox (and that have already been copied to the trash folder). I suppose that's what expunge means - to get rid of those messages.
Alpine however is much more consistent, but now I have this (nonproblematic) feature:
I have a filter in Alpine set up to also move 'expunged' messages to Trash upon expunging them. Otherwise Alpine would just delete everything with no fallback. It would be gone = gone.
So if I delete a bunch in Thunderbird, and then open and exit Alpine, all of those deleted messages are now featured twice in the Trash, once copied there by TB, and once copied there by Alpine :p.
I don't know what the actual point is of keeping "deleted" messages in any folder if you have already visually indicated they are gone and they have been moved elsewhere. Is there some sort of "undo" mechanism involved? There is no action anywhere in TB that *I* can see that would benefit from something like this?
For Alpine, the "D" (delete) action only flags messages. Then, upon exit they are usually expunged. With my custom filter, they are expunged and saved to Trash. But TB instantly deletes and moves them, which is fine by me, but I don't understand why it doesn't clean up behind itself.
Is there any use case where NOT expunging these 'flagged' messages has benefits, in terms of Thunderbird?
And I have heard people talk about auto-compacting, but why did such a thing never happen?

Answer 1

re: TB marks messages for deleted both in its offline copy and in the IMAP proper,
Any changes such as deleting a message, will mark that email as deleted.
In thunderbird this will move the email from the Inbox view (''Note this will depend on the deleting method - see below'') and place it in Trash, but in reality it is still in the Inbox folder just 'marked as deleted'.
As the Server folders and TB folders are looking at the same folders, this will show up on the server as 'marked as deleted'.
Thats because they are one and the same folders.
In Thunderbird, you can select to manually Compact all Folders:
File > compact folders.
This compacts all folders, removing all 'marked as deleted' emails in Thunderbird..
when synchronised folders connect with server, those emails are deleted off the server.
You can also choose to manually compact any single folder.
right click on folder and select 'compact'.
There is also an auto compaction function which is initialised when the selected threshhold has been reached.
This is set up here:
Tools > Options > Advanced > Network & Disc Space tab
select the checkbox 'Compact all folders when it will save over eg: 20MB in total.
click on OK.
Note if you have been manually compacting or expunging the Inbox on exit then this may not get triggered as often or at all if the threshold is not reached.
IMAP mail accounts also have the additional option to Expunge emails on the Inbox on Exiting Thunderbird..
"Tools -> Account Settings -> Server Settings -> Clean up ("Expunge") Inbox on Exit', which will compact the Inbox folder when you exit Thunderbird.
note this only occurs on the Inbox.
If you check "Empty Trash on Exit" it will also empty and compact the Trash folder when you exit.
However, some methods of deleting in IMAP mail accounts can disable this auto expunge. see below.
'''IMAP mail accounts also have different choices when it comes to deleting.'''
* http://kb.mozillazine.org/Deleting_messages_in_IMAP_accounts
Tools -> Account Settings -> Server Settings for the mail account
"When I delete a message" has choices for "Move it to the Trash folder", "Mark it as deleted" and "Remove it immediately".
Your choice of method of deleting may explain why you are not always seeing auto expunging.
suggest you try option: "Move it to the Trash folder".
Otherwise you will need to use the folder 'compact' options explained above to compact the folder.