I suggest in the synchronization pane to select under 'Disk Space' to synchronize only the past five days. that should let all headers be visible, but not download the content unless selected. The headers, at minimum, must be downloaded so Thunderbird can present message list to you.

@David

I have changed this to 5 days as mentioned. Will this automatically delete any messages stored locally that are older than 5 days? or will I need to manually delete them?

This is what I have currently. All folders have been unticked in Synchronization & Storage -> Advanced I have changed the "Synchonize the most recent" from 30 days to 5 days My imap folder has 22.1gb, the exact same as it was previously

These steps will get you off to a fresh start: - exit thunderbird - in the profile, locate the imapmail\<accountname> folder and delete the inbox and other folders, including the msf entries. - now, restart thunderbird and it will repopulate according to your new settings. since the acount is IMAP, no content has been lost since the 'real' content is on the server. you actual storage on PC should be much smaller - do report back on results.

If you are unfamiliar with working within the profile, here are steps to get there: - click help>troubleshootinginformation - scroll down to 'profile folder' and click 'open folder' - exit thunderbird, you are now in the profile and the imapmail folder should be visible.