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I'm using Thunderbird with Gmail IMAP and there's this "Subscribe" option for folders. What does that mean?

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    This is now a somewhat dated question and Thunderbird has changed a bit since it was asked. To get more up-to-date information see Thunderbird and Gmail. Modern versions of Thunderbird handle gmail as a special case because its unique implementation of IMAP. For information on how Thunderbird handles standard IMAP email, see Mozilla's support page about IMAP Synchronization. – martineau Apr 15 '13 at 19:41
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The folders you subscribe to are the ones displayed by Thunderbird. It subscribes every folder it finds when you first setup the account and any folder you add through Thunderbird is automatically subscribed as well. I guess that option is there so if you were to add a folder through another application or a webmail-ish app and you wanted that new folder to be displayed as well you can just turn it on.

Maybe this link will clarify things. Here's a quote:

Your IMAP email client (eg Thunderbird) will display the folders in your account to which you are "subscribed".

Generally, this will be all of the folders in your account, and only the folders in your account. However, in some situations you may find that there are folders in your account that do not appear in Thunderbird, or folders that appear in Thunderbird that are not in your account. This can be fixed by subscribing to (or unsubscribing from) the offending folders.

  • How is this different from going to File->Offline->Offline Settings->Select folders for offline use? This doesn't seem to be the same as subscribing (at least as suggested by the UI), so I guess it must have a different meaning. – hsribei Oct 27 '09 at 18:34
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    I edited to clarify. If you were to login to your IMAP account through a web portal and create a folder then Thunderbird would not even display it until you subscribed to it. – djhowell Oct 27 '09 at 18:53
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    Don't forget that you may also want to turn subscriptions off - one of the benefits of IMAP is that state is (in theory) maintained on the server so you might not, on a mobile device or on a laptop (particularly a netbook) want to subscribe to folders that potentially have a high volume of traffic – Murph Oct 27 '09 at 21:14
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This is due of the peculiarities of the IMAP protocol. Strangely enough, the authors of that protocol not only thought about email when they dreamed it up, they also thought about usenet and newsgroups. The newsgroups of the usenet form a large hierarchy with thousands and thousands of groups. The original aim of IMAP was to support having the complete hierarchy available through your IMAP account. Of course, you wouldn't want all those groups to show up in your mail client, thus the concept of folder subscriptions was added to the protocol.

If I remember correctly, Thunderbird also has an option to simply show you all your available folders without going through the motions of subscribing to single folders. It has nothing to do with offline use or synchronization.

If you want to know more about the IMAP protocol, check out the corresponding Wikipedia article. You can also try to read the corresponding RFC which makes up the protocol and which is full of examples concerning usenet newsgroups (and has only a few examples that relate directly to email).

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There are already a couple of very good answers to this question (which I’ve upvoted) but I thought I’d complement them with an answer detailing the relevant commands specified by the IMAP protocol, RFC 3501.

Subscribing to folders

In IMAP, subscriptions are used as a way of marking which folders should be displayed by IMAP clients. Some system folders may provide functionality but not contain emails that are of interest to the user, e.g., an Exchange server contains folders such as Contacts, Journal, Sync Issues).

Two IMAP commands are used for listing IMAP folders:

  • the LIST command lists all folders.
  • the LSUB command lists only subscribed folders.

The output of the LSUB command is affected by 2 commands:

  • SUBSCRIBE adds folders to the list displayed by the LSUB command.
  • UNSUBSCRIBE removes folders from the list displayed by the LSUB command.

See What is the use of Subscribe and Unsubscribe commands in IMAP4? for examples of the effects of the subscription commands.

Note also that folders are marked as subscribed on the IMAP server so that all clients see the same list of folders. See Is IMAP (un)subscribe meant to work across mail clients?.

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I believe that it allows you to subscribe to a folder on the server, there by syncing the contents of whats on the server with what's in the Thunderbird client.

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2017-01-26: Thunderbird 45.5.1: The previous answer, about starting at File, was the key for me.

From a particular account: File | Subscribe results in a list of folders, then: Gmail then from a list of folders: check or uncheck the folders desired or not desired. In my case, it was eliminating Important and All.

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