Have you considered moving some of the messages to another folder to reduce size?

I tried that - I split that folder into two pieces and compressed both. Made no changes to either.

30 minutes later it spent 15 minutes compressing one of the two pieces.

FYI - my local folders are on a network drive (there is a specific reason for that)

Thanks

Jonathan

er :FYI - my local folders are on a network drive

So not in the default location. I've come across various issues when profile or parts thereof are on network drive.

If file is on network /cloud then that involves a more complex operation as file must be accessed/downloaded first.

Compacting means opening original file, creating a new file, reading original file, copying each email into the new file providing it does not have any 'marked as deleted' markers - those emails are ignored. When finished, old file is deleted (eg: Inbox) and new file takes the name of deleted file eg: 'Inbox' and the 'Inbox.msf' file is deleted and recreated based on the updated Inbox mbox file.

You could switch off the auto compacting. Then manually compact any folders from which you either delete or move emails eg: Inbox, Drafts, and Junk/Spam tend to be the most frequently used folders that come into that category. If using Imap then Inbox can be set up to expunge emails on Exit of Thunderbird - Account SEttings > Server settings under 'Message Storage'

OR switch on auto compacting but force it to ask you first which means you could delay the action whilst you complete a task etc.

Advise - never allow any single mbox text file to be larger than a suitable size which a simple text editor program could open.

Advise - do not click on folders and view emails ect when compacting as it may interupt the process. Trying to access a file when it's being compacted is no adviseable. Never exit Thunderbird.

Personally, I prefer to have compacting set to tell me when it's required but I have control over when to use it. Then I go into Offline mode to stop incoming traffic, I set up to do compacting and walk away and make a cuppa. By the time I get back, all is done and I go back into online mode.