Two upfront thoughts: - if gmail account, removing subscription to allmail will reduce significantly - check in profile folder for account to see if any files labled NSTMP.... if so, those represent failed compactions and can be deleted.

For more suggestions, posting screenshot of account's folder in imapmail\<accountname> may prove helpful. Thank you

Thanks to Wayne for fixing the formatting. For David's response - they're not gmail accounts so I can't change anything there. I'll check for nstmp files when I'm back in the office - that certainly could be useful, and if I do find lots of them it'll be easy enough to automate the cleanup. I'll also get the relevant screenshots while I'm there. Thanks!

Just an aside. Check the account settings have expunge inbox on on exit checked and that the user actually restarts the application frequently. Many issues come from expunge failing to execute.

In account settings change the delete action to just mark it deleted and restart Thunderbird that used to, and I am assuming still does, displayed all the deleted messages in the folder pending expunge with a strikethrough. That should offer some insight into if that is a "real" issue in this instance. My guess is it is.

Compacting forces an expunge server side as a part of the process although I have a sneaking feeling that was dropped in the last couple of versions instead relying on the preference mail.imap.expunge_option and Thunderbird actually closing down to send the expunge message.

So;

• Check in the Config Editor that mail.imap.expunge_option is not set to 0
• Check the user actually shuts down Thunderbird to allow the expunge housekeeping to run.
• If the user has a laptop, ensure they actually shut it down before disconnecting from the network. It may be that Thunderbird is still running in the background when they literally pull the plug so housekkeping fails daily.
• Check the user has not gone and set Thunderbird to check mail every minute or two, basically nothing else will ever happen and all mail may never be downloaded in that scenario. Once the timing gets more frequent that every 5 minutes often things break down. Especially if the user is not connected to the server with a gigbit fibre connection.

Have a similar problem ...

   "Check in the Config Editor that mail.imap.expunge_option is not set to 0"

What should this be set at?

Ok, I'm back in the office and able to provide a few updates.

• I'm not finding any NSTMP files either through Windows search or a command prompt using "where /r" - I don't think that's the issue.
• mail.imap.expunge_option was set to 0 - I have changed it to 2 - see the wiki for more information. I have also enabled expunge_after_delete. I had thought expunge referred to server-side deletion, whereas local cleanup is carried out by compacting the folders, although I am willing to be corrected!
• Both the client PCs with the problem are desktop PCs rather than laptops.
• Every account I've checked has the check mail frequency left at the default of 10 minutes. I'm pretty sure I managed to get IMAP IDLE working on the mail server. Oh, and we are on gigabit fibre.

Thanks for the suggestions, Matt!

It appears to me to be something going wrong when compacting the folders. I located an outsize mbox on one of client PCs that showed up as on disk as 11.4Gb, while being listed in Thunderbird with "Show folder size" enabled as 704Mb, which matches the size for the maildir folder on the mail server. When grepping for "^Subject:" to get an approximate message count, the maildir folder returns about 700 matches, which corresponds to the number of messages visible in Thunderbird. The 11.4Gb mbox returns almost 12000 matches. I have tried manually compacting these folders with no effect; if I try using the "Repair Folder" option, Thunderbird appears to wipe the mbox completely and then re-download all messages. As I said above, I'm not finding any NSTMP files left over from failed compacting.

Thanks for the help so far - I think I can see what's going on, even if I don't know why yet. If the extra info is useful, I'd like to try to pin this one down.