Expanding on what phoenix said on 6/15:
The IMAP protocol, which Thunderbird is speaking, has two separate commands for "flag for deletion" and "expunge." Back in the day when each mailbox (or folder) was one large file, expunging messages from the middle of a mailbox was a very expensive operation, because it meant rewriting the whole file.
Thunderbird calls the expunge operation "Compact Folder."
Nowadays, most email servers use some variation of file-per-message or database storage, so the original technical reason for the split delete/expunge process no longer applies. Nowadays, pretty much everyone considers "Move to Trash" a far more user-friendly paradigm than mark for deletion/expunge. But the IMAP protocol remains the same. There simply is no "Move to Trash" function. IMAP requires you to copy the message, then mark deleted in the original folder, then expunge the original folder.
The discrepancy in message counts in Drafts (and sometimes elsewhere) is due to Zimbra web's counting, but not showing, messages in the "deleted but not yet expunged" state.
Zimbra web has no way to expunge deleted messages because like most enterprise packages, it has moved entirely to the "move to Trash," "Drafts instantly move to Sent" paradigm. Any cleanup of expunged messages needs to be done by an IMAP client. Thunderbird does "lazy" expunge, only after a certain amount of time or messages have passed. |