Gmail maintains multiple labels, all pointing to a single message. If an email client is asked to reference each as a folder, a copy appears because the email client must download the message to the PC in order to be processed. Since some of Gmail's labels are duplicates, such as 'all mail', 'important' and some others, I always recommend setting up Thunderbird to use only the folders needed, such as inbox, sent, trash to eliminate the need for Thunderbird to maintain duplicate copies. Another reason for my recommendation is to avoid confusion. For example, it is not uncommon for users to post that they deleted a message from inbox, hoping it was saved in Allmail, but since that is the same message, it is deleted from both.

Thanks for your answer, David. I understand your recommendation about excluding labels in TB. But it doesn't finally serve my goal. As I said, my GMail storage is about 12 GB / 15 GB. All my mail have labels, at least one to classify them in a primary category, and sometimes two or three to attach them to special contexts. For example, mails with my bank are classified with a main label, and the same messages also have labels to say they relate to my house or my car, and so on. Now I'd like to use the IMAP synchronization to clean some old mails and free part of the storage. And I need this classification to backup in local folders and remove mails depending of their age, first, but also according to their other labels. If labels are not available in TB, I must sort and classify all my mails from scratch : years of work :( ! IMHO, TB should consider labels, as GMail does, as simple meta information added on mails to organize them according to several orthogonal axes... Isn't any way to synchronize all my mails, without multiplying copies and disk storage, while maintaining all my historical classification ? Thanks again.

I am unaware of any feature in Thunderbird that is equivalent to Gmail's labels. Since you appear to have established this framework within Gmail, that may be your best tool to address your concerns. However, others here on the forum may have other suggestions

Gmail has only one copy of an email and it's always stored in 'All' mail' Anything else you see in gmail is displayed because a type of tag which gmail call a label has been applied to the email stored in 'All Mail' This means you see a load of virtual folders containing emails and it also means gmail can display one email in various other virtual folders.

Thunderbird does not have any concept of labels - it is purely used by gmail as a way to show emails in a more traditional view. Thunderbird downloads whatever it finds on server for whatever folder you have subscribed to see. Thunderbird stores each email in each folder as a separate copy. If you were to delete one of the copies stored from one of the folders and that email is correctly put into the gmail Trash folder, gmail reads this as delete main copy on server, so it deleted from the 'All Mail' folder and therefore all labels get deleted at same time. This means when you synchronise your folders you will discover all of your copies from all of your folders will get deleted. If you were to delete one of the copies using 'Shift'+'Del' key, thus by-passing the gmail Trash then gmail reads this as remove a label. So, any other copies in any other folders remain in those folders.

Thanks for your detailed explanations, even if they do not give me many solutions to retrieve all my Gmail accounts in TB with their label systems.

I've never been able to look at the source code of OSS like TB as a developer, and I have no idea about difficult is or isn't adding features. So my question is probably going to bother some of you.

I use TB for my personal use. For my professional mails I've been using MS Outlook for years (with almost all its versions), and with OL labels provide a similar way to classify mails (and other object in calendar, tasks, and so on), and without replicating them. With OL, if I'm not wrong, labels are quite simple and don't allow to create complex trees as Gmail does. Not sure, but in my memories Notes also had some kind of labels for mails.

So, would it be very complex adding some kind of label feature to TB that could work as metadata, and do not lead to replicating X times the same content ?

I know that sometimes I have some weird ideas, but I'm almost sure that other could use such features, specially when using Gmail accounts.

Thanks again.

re :So, would it be very complex adding some kind of label feature to TB that could work as metadata, and do not lead to replicating X times the same content ?

Thunderbird can only download a copy of whatever is displayed in that label, so whilst gmail is displaying a virtual copy in the Inbox 'label - the download of a copy from server Inbox to imap Inbox is a real email and gets stored in a real mbox file. Nothing virtual about the download - it's real.

Thunderbird does use 'virtual' folder displays. The 'Unified Inbox' folder display is a virtual folder that displays all emails currently stored in various Inboxes for various accounts. You can also create a search result as save as a virtual folder. But those virtual views depend upon a real email being stored somewhere.

If you have already chosen to have an email in 'All Mail' to have two different labels, so that Gmail can display that email in eg: Inbox and Important, then in Thunderbird, you will see a copy in 'Inbox' and in 'Important' assuming you have chosen to subscribe to see 'Important'. Neither copy is virtual in Thunderbird because you cannot download virtual emails. But depending upon how you 'delete' either of each copy depends upon how gmail perceives that instruction.

You need to decide whether you want gmail to sort emails automatically by applying 'labels' or whether you want to control where you see emails.

I have stopped gmail from making decisions regarding my email. Nothing automatically gets put in any 'Important' folder etc by gmail - I have all incoming going into 'Inbox' and I sort it into suitable folders. If I subsequently want 'virtual' copies in my own 'Important' folder then I create my own Search Messages filter for specific emails and save it as a Search Folder which I call 'Important'. Then I have a copy displayed in a folder and it's virtual. But because it is a search result folder it will not get put into gmail.

You have to jump through hoops when dealing with Imap accounts and this means you have to understand that the server and imap folders are one and the same, it's just that in gmail it has virtual labels and in Thunderbird it is real files - otherwise you would never be able to backup something that does not exist.