re :I run Thunderbird as my email and I archive old mail to local folders,

I presume you mean the 'Local Folders' special account. I presume you have an imap mail account - but do you download full copies of all emails or download just headers? When you need to get copies of emails into the 'Local Folders' account, I suggest you always use 'Copy to' and not 'Move to' because if something goes wrong when moving stuff off the server you could lose emails, but if using Copy to' then you can always try again. Can you read your emails in Offline mode ? If no then you have not got fully downloaded copies.

re : I have a suitable set of .MSF files (over 100, 000) synced to Dropbox. Files which have a .MSF extension. They are index files which typically contain headers, info on size and info on that folders various column headers status etc. They are not easy to read. They do not contain any email content. If you believed backing up .msf files is backing up emails then you are mistaken.

If you have downloaded full copies of emails then they are saved in mbox text files which do not have any extension. I've attached simple example images below - any file with type 'FILE' is an mbox file containing emails. Any file with type MSF is an index file. If you see folders with a .sbd extension that means the folder may contain subfolders - more mbox files and index files.

re :Is it possible that for some reason Thunderbird does something to the files (for no apparent reason)?

Whenever you receive, send, delete or move anything, mark as read, tag, alter column header display etc etc or compact folders to remove all old data (anything moved or deleted) then files get updated. So in effect, many files get updated very frequently.

If you use an imap account, those files may also get updated whenever there is a synchronisation with server.

Dropbox synchronisation to Microsoft Account server is external to Thunderbird and not controlled by Thunderbird. It frequently synchronises especially if any file gets updated.

Dropbox can be useful especially if you want to use it to synchronise with another computer. But please remember, it's only useful if you have access to it via password. If you get locked out for any reason you may find you locked out from anything on that server - it could be a real problem if you are relying on it. I would always advise you have a manual l backup to something that does not rely on the internet or access to a server. eg: an external disk drive.

Dropbox info: https://help.dropbox.com/sync/check-sync-status

Hi, Yes, my Thunderbird client is connecting to Gmail with imap. I am using the Local Folders account to download full copies. I just have a folder hierarchy under the Local Folders a/c where I drag and drop emails to keep them as offline copies.

I can understand if I open any of these, Thunderbird might 'touch' the files and so Dropbox would sync them. But if I haven't looked at a subfolder, I would not think Dropbox would realise.

I noted that this 'full sync' doesn't happen on every open of Thunderbird / Dropbox.

One thought - I use Avast Cleanup Premium to keep safe, and it cleans up things every week, including temp files, cookies etc and cleans caches. Could it be that this makes either Thunderbird or Dropbox forget what has been synced?

re :One thought - I use Avast Cleanup Premium to keep safe, and it cleans up things every week, including temp files, cookies etc and cleans caches.

I would make your Thunderbird profile exempt from scanning or performing any cleanup on anything in Thunderbird. You can manually clean up cache from within Thunderbird via Settings > General - Disk Space section. You are fortunate if you have not encountered any issues.

Gmail needs cookies. Session files need to retain data. Many files get updated everytime you open and use Thunderbird.

I am using the Local Folders account to download full copies. I just have a folder hierarchy under the Local Folders a/c where I drag and drop emails to keep them as offline copies.

If you are 'moving' an email using drag and drop from an imap account to a folder in Local Folders, then you are moving that email off the server which means there is synch with server for every email you move, as the email being 'moved' or rather 'deleted' from an imap folder that means it is bypassing the Trash, so gmail is probably just removing the label and archiving those emails in the 'All Mail' folder.

Moving an email from an imap account into Local Folders does not download email. The email must already be fully downloaded in order to keep a fully downloaded copy in Local Folders. Fully downloaded emails are stored in mbox text files and they do not have any extension. Those settings can be applied on a per folder basis in Account SEttings > Synchronisation & Storage. If you do not have fully downloaded copies in imap folders then you are only moving the headers into 'Local folders' and they use .msf files. Why am I mentioning this ? - because you mentioned backing up a load of .msf files to dropbox. You did not mention you were backing up mbox files which have same name as folder eg: 'Inbox' but they have no extension - the index files that store email headers and other data about the folder will be called eg: Inbox.msf Are you backing up the mbox files ?

Have you tested to see if you have fully downloaded emails ? Go into 'Offline' mode, can you still access emails AND read their content in imap folders and very importantly in Local Folders?

I would always advise that you should 'COPY' - right click on highlighted emails and use 'Copy to' and choose folder. It's less risky than 'moving' - however, if you access gmail webmail account and locate all those emails still in 'All Mail' account, then in case of loss due to something going wrong with server synch, you could re-issue them with a label.

Synch with imap account can be forced by gmail when new mail arrives on server because gmail use an IDLE command. If you perform any move or deletion of an email in an imap folder, there is synchronisation with server. If you have set up gmail to auto check for emails then there is synchonisation with server. Any email that gets opened, tagged, starred, moved etc etc the msf files get updated and there will be synchronisation with server. If you sent an email, it may have initially been saved in Drafts and after sending stored in Sent. That involves synch with server, to remove the 'Drafts' label when it is deleted from Drafts and apply 'Sent' label, so forcing a synch with both of those folders and that involves marking any moved or deleted email in an mbox files and also updating the index file. It will also involve updating a load of other files in the profile so a record is kept. If you subscribe to see 'All Mail' then that file will be almost constantly being updated and therefore forcing a synch with server.

As you are moving a load of emails then that means you also have a lot of 'marked as deleted' emails in folders such as Inbox, Drafts, Spam and any other fodler where you move or delete an email. To maintain a healthy Thunderbird, those mbox files need a cleanup and that involves compacting (not to be confused with compression). This also means the .msf files will get updated. Many people have the Inbox set up to expunge on exit - that means compacting to keep it tidy.

Thanks for all the help.

For now, I will just wait and see if it still syncs all files over the next couple of weeks.

Assume all is fixed, I will post again if it still syncs.