Imap mail accounts see a remote view of the server. You subscribe to see folders. Headers only are downloaded. You would click on an email and TB would retrieve the email from server and store in a temporary cache to faciliate viewing. These emails are not stored in Thunderbird and will not be visible if you went into offline mode. The emails are stored on the server. If you sychronise subscribed folders for offline use then a copy of the emails on the server is downloaded and stored in the Thunderbird profile folder. This means you can view those emails in offline mode, but those folders will still synchronise with server when in online mode. If you had already created the imap mail account and synchronised emails for offline use which were older than 6 months and then went into 'Account settings' > 'synchronisation & Storage' to change settings to 6months, it will not remove previously synchronised for offline use older than 6 months emails. But it will sychronise only 6 months from then onwards. You should still be able to read previously synchronised for offline use emails older than 6 months emails as a full copy had previously been downloaded to your Profile folder. If you had already created the imap mail account, but had only subscribed to see folders - not synchronised them for offline use, then only headers would be downloaded. There is no copy in TB. When you then change Account Settings to only synchronise last 6 months, then TB would do that - only show content for those emails. Email headers older than 6 months are now not able to get retrieved/synch with server because you switched it off for anything older than 6 months. This is what I believe has occurred in your case. Please note that previously (older than 6 months) downloaded headers or emails synchronised for offline use are not deleted. They are not being downloaded again. I trust this explains what has occurred in your case.