(In reply to Alex Ihrig [:Alex_Ihrig] from comment #0)
Steps to reproduce:
1.) Create an IMAP account in Thunderbird
2.) In Account Manager choose the option to "Mark as deleted" for deleted messages
3.) Delete some Messages - they should be marked as deleted.
Do NOT compact the folder (they should not be expunged at the moment).
4.) In Account Manager change the option from "Mark as deleted" to "Move to Trash folder".
Now in Folder Pane all deleted messages disappear.
The issue with these disappaering messages is, that they are not visible but they fill the IMAP quota (size and/or number). This is a major problem, if many messages are marked as deleted and the number exceeds limitations of your provider (e.g. T-Online has a limit of 1000 messages per folder).
Seem like a pretty drastic limit of 1000 messages/folder. Does T-Online also limit the number of folders (mailboxes) you can have and the amount of storage in a folder? FYI, tb latest beta has an improved quota display that will show any and all types of quota (called quotaroots) that the server defines for a folder, e.g., Storage (in Kb), number of messages and number of mailboxes etc. I'd be curious to know if T-online actually shows these quotas. (Well, not sure it's in beta but definitely in daily.)
The problem escalates unnoticed if you use multiple IMAP clients (or multiple Thunderbird installations) with different settings. If one client only marks "deleted", the other clients Users cannot see that the mailbox is full.
The "improved" quota should be visible on any tb client so maybe this will help?
The problem becomes even worse if the deleted marked messages are not displayed in Webmail. In the case of T-Online, a quota of, for example, 20% is displayed in webmail. Via IMAP, the quota is then 100% (exceeded), although you may only have 10 visible messages in both the webmail and Thunderbird.
Does T-Online support imap MOVE or UIDPLUS capability? If imap MOVE is supported, any message deleted and "moved to trash" will actually be expunged from the source folder by the server. If move is not supported but UIDPLUS is supported, messages deleted and "moved to trash" will be "copied to trash" and then UID expunged by tb command from the source folder, so this effectively simulates MOVE. But if neither MOVE or UIDPLUS is supported by the server, the deleted message will by copied to trash and left marked as \deleted in source folder and invisible with "move to trash" selected. If "just mark as deleted" is then selected, the invisible messages appear with the red X and strike-through. (You may need to record an IMAP:5 log to know the supported capabilities: https://wiki.mozilla.org/MailNews:Logging.)
Compacting folder would resolve the problem. But there are cases, where the compacting fails (today we had a support request in german forums with 2 undeletable / uncompactable messages of 5000 messages in Trash. Because of not beeing able to compact, all 5000 messages remained in Trash. Due to T-Onlines limit of 1000 messages the whole mailbox was blocked. The user had no chance to find the issue because of the invissible 5000 messages in this trash folder.
This sounds like a T-Online bug. Do they not send you a warning email or SMS when the (very strict) quota is close to being reached and possibly locking the mailbox?
I'm not clear on how 5000 messages got into Trash and all marked as \deleted and invisible (or maybe just two?). Tb always move/copies to Trash and leaves the \deleted flag off so they are not invisible sitting in Trash. Then when you empty trash the messages are all marked \deleted and then the Trash folder is expunged, permanently deleting them all.
This leads to incomprehensible Quota Exceeded messages, which can only be corrected by chance after a long search. The providers Support Team was unable to find these invisible messages.
Reading again, not sure what you mean by "2 undeletable / uncompactable" in trash. Did tb cause this?
What I would like to see improved in Thunderbird:
Thunderbird should always display messages marked as deleted via IMAP in the folder pane - even if you select the option "Move to trash when deleting" in the account settings because the messages previously marked "deleted" or marked as deleted in another client are still available . There is no reason why these still existing messages should be hidden!
Maybe I'm missing something, but you can just change the delete mode back to "just mark it as deleted" and you will see any invisible messages (marked as \deleted) with the red X and strike-through. But I think you are saying you want to see this always.
P/S: I vaguely remember a request like this before in another bug report but can't locate it.
Edit: This is the same request as bug 1477557.