From MozillaZine Knowledge Base

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

IMAP (Internet Message Access Protocol) is a network protocol used to access e-mail messages while they are still stored on the server. It supports a client server view of the world. This means that IMAP lets you access messages stored in a remote folder as if it were a local folder. This includes copying, moving, reading, and deleting messages. Thunderbird by default also keeps copies of all of the messages in your remote folders in "offline folders", stored on your hard disk. When you are working online you see the contents of the remote folders. When you are working offline (File -> Offline -> Work Offline) you see the contents of the corresponding offline folder instead. You can disable this feature in Tools -> Account Settings -> Synchronization & Storage.

Most users are more familiar with POP (Post Office Protocol), which supports a download-centric view of the world, i.e. you read a message by downloading it from the server's inbox folder rather than remotely viewing it wherever it's stored. IMAP is a newer and more powerful protocol. It lets you use all of the folders (just like webmail) while POP accounts are limited to just the inbox folder. However, many ISPs prefer to support POP instead because most users are used to it and they can provide a smaller mailbox (since users will be downloading messages and keeping none or only some stored on the server).

If you have only used POP accounts you may be used to deleting a local copy of a message, knowing that the original is still stored on the mail server (and accessible via webmail). That isn't true with IMAP. That is why the folder in the IMAP account is called a remote folder. Whatever you do to the remote folder affects the corresponding webmail folder. It also affects the corresponding offline folder, if it exists.

Thunderbird, the Mozilla Suite, and SeaMonkey support IMAP accounts. Some of the main features are:

  • It's very useful for roaming users since the messages are not (only) stored on whatever PC you're using. The main limitation in accessing your messages from another PC is that most email providers don't provide a way to share address books. There are several extensions that support that.
  • If your profile folder is corrupted or your computer breaks down, Thunderbird will be able to access and re-download your messages from your service provider.
  • You can copy or move messages and/or folders between local PC folders and remote IMAP folders at will.
  • You can automatically check for new mail in any IMAP folder using folder settings.
  • You can control which folders are visible. You "subscribe" a folder to make it visible, unless it's subscribed by default, or you disable "Show only subscribed folders". With Thunderbird you right click on a remote folder and select "subscribe" to get to the dialog box that lets you specify which folders should be subscribed.
  • You can use the IMAP account from multiple email clients at the same time. The closest equivalent to this feature with POP is to configure "leave messages on server". However, that only lets you access the inbox.
  • IMAP supports public folders, which can be used to share selected folders with other IMAP users. Typically you'd have to use a webmail command to manage which users can access that folder (by setting the appropriate ACL). This is optional functionality that many commercial IMAP email providers don't support.
  • In older versions of Thunderbird, IMAP by default only downloads the message headers to the hard disk. It does not create an mbox file like POP accounts have. This means that if you open the message again and it's not still cached in memory, it will be fetched from the IMAP server again. Thunderbird supports preferences that avoid fetching attachments until you open them.
  • When you delete an IMAP message, you're really just setting a message flag. Because of this, Thunderbird lets you specify whether deleted messages should be removed immediately, flagged, or moved to the trash folder. This also means that another email client might let you see the "deleted" message until you compact that folder. Some email clients hide messages deleted by other email clients, some display them normally, and some display them with a line through the headers (to indicate it was deleted by some other client). It's an implementation-specific decision.
  • It's easier to migrate to a different email client (if it supports IMAP) since you don't have to move or convert the messages from your computer to a different computer.
  • IMAP supports an offline mode which can be used to keep a local synchronized copy of a remote folder. A change to either folder affects the other folder the next time you synchronize. Some users may find it easier to use a message filter to automatically make a copy of a new message instead, if they're only concerned with the inbox.

Many IMAP email providers provide additional functionality such as server side filtering, the ability to automatically fetch and merge email from several external POP accounts (or hotmail) into your IMAP mailbox, aliases, plus or subdomain addressing (which are usefull for creating email addresses on the fly to help deal with potential spam), and run server based tools such as SpamAssassin (which adds headers useful for spam filtering).

AIM (AOL) provides a 2GB mailbox, though its features are non-standard. Gmail has a 15GB mailbox, with some quirks such as a optional All Mail folder. GMX has a 5GB mailbox, with standard features. Yahoo and have standard features and supposedly provide a unlimited size mailbox. Outlook/Outlook Express, Thunderbird/Sea Monkey and Mulberry are considered the email clients with the best IMAP support.

IMAP is mainly used for mail messages but it's possible to use it for other types of storage. For example, one email provider also lets you store files in remote folders and configure web access to them. It essentially adds a WebDav server that accesses the files in the remote folders. You could use this to offer a home page for example.


Thunderbird specific features

  • A saved search folder for an IMAP account supports both online and offline searching. However, the name is misleading - it basically lets you create a virtual folder. "Online searching" just means that the virtual folder is updated every time you open it. IMAP accounts don't support a Global Inbox, but you could use a saved search folder to emulate one, but this will be empty unless you are online.
  • Offline folders typically are synchronized when you switch to working offline. Thunderbird also supports a option to automatically download messages for offline use whenever you open a folder or it detects new mail.
  • Thunderbird 3.x supports message synchronizing in Tools -> Account Settings -> Synchronization and Storage. It automatically synchronizes a different type of offline folders while you are online. The main difference is that they're not just for working offline, Thunderbird will load messages quicker by using the downloaded messages as a cache. It currently stores them as mbox files (just like offline folders) though they plan on changing the format later on.

Push support

"Push e-mail" means that when a message is delivered to the mailbox you want to get immediately notified of it. Thunderbird provides that if your IMAP server supports the optional IDLE command. The IDLE command eliminates the need for a e-mail client to poll for new mail - the mail server automatically notifies the e-mail client whenever there is new mail. "Push support" is sometimes mis-interpreted as IDLE support but it is actually proprietary Push-IMAP enhancements for mobile devices such as cell phones developed by Oracle. It can use IDLE, SMS or WAP Push to provide the notification.

The IETF has defined a standard called the Lemonade Profile that is meant to replace Push-IMAP. It includes other features such as the ability to forward a message without downloading it and the ability to quickly re-sync a connection. The Lemonade for Mobiles web site has more information on what e-mail clients and servers support Push e-mail and/or Lemonade. While Lemonade is mainly thought of for cell-phones it is also useful for laptops on trains and planes, and PC's using satellite links. Unfortunately it wasn't even considered for Thunderbird 3.

See also

External links


Name space

Mozilla documentation


New functionality being developed

Possible replacements

Discussions about a possible replacement for IMAP. Having the protocol also support address books, calendars and submission is being considered. Several people active in the tb-planning mailing list are involved, but no Mozilla employees.