From MozillaZine Knowledge Base
This article was written for Thunderbird but also applies to the Mozilla Suite. The main difference for the Mozilla Suite is that it uses a different menu to get to the account management settings
May help prevent problems
- "Tools -> Account Settings -> Server Settings -> Advanced -> IMAP -> Maximum number of server connections to cache" defaults to 5. Courier IMAP servers default to 4 connections per TCP-IP address. This mismatch can cause apparent hangs, data loss, and messages to be downloaded instead of just headers. If you have one Courier account you need to lower the Thunderbird setting to 4 or less. If you have two Courier accounts you need to lower the setting in each account to 2 since it's 4 connections per TCP-IP address, not per account. If you don't know whether you have a Courier IMAP server it doesn't hurt anything to lower the values.
If you have a local network with several PC's connecting to the Internet through the same router/modem (e.g., DSL or cable) check whether each PC uses the same same TCP-IP address to access the Internet (NAT). In that case you could quickly reach the limit even if each PC is using only one account. One way to check is to go to a web page that reports what TCP-IP address was used (such as this one) from each PC.
- Thunderbird defaults to only showing subscribed folders. That's a way to manage the clutter when you have a lot of remote folders and you don't want to see all of them in the folder pane. You subscribe a folder by right-clicking on the remote Inbox, selecting "Subscribe", expanding the Inbox tree in the window, and subscribing whatever folders you want. You can configure it to show all folders by unchecking the checkmark at "Tools -> Account Settings ->Server Settings -> Advanced -> IMAP -> Show only subscribed folders".
- If you have problems creating (or seeing) remote child folders you might be using a IMAP server that doesn't support both messages and child folders within a child folder. Try unchecking "Tools -> Account Settings ->Server Settings -> Advanced -> IMAP -> Server supports folders that contain subfolders and messages" and use webmail to move the messages out of that child folder.
- It's usually best to leave the IMAP server directory field empty, check the checkmark for "Allow server to override these namespaces" and to use "INBOX." (the trailing period is critical) in the personal namespace field. There are some IMAP configurations where that's inappropriate but it's not as common as most people think. If the namespace is not configured correctly you will typically see multiple copies of a child folder, child folders as peers to the Inbox, or be unable to create child folders. Usually this occurs when a email provider doesn't support creating folders that are a peer to the inbox (rather than being a child folder) and somebody tries to edit the namespace to workaround that. Thunderbird normally displays the Inbox folder as the root, with no peers.
- If your email provider doesn't support creating folders that are a peer to the inbox one workaround would be to create a child folder somewhere thats normally hidden from view and then create a Saved Search folder (aka virtual folder) whose search criteria selects all messages in that child folder. That virtual folder can be a peer to the inbox. While the messages are still physically stored in the child folder you can use the virtual folder as if its a real folder. It will just have a different icon in the folder pane.
- The IDLE command is a extension to the IMAP protocol that lets the IMAP server notify Thunderbird when there is new mail by sending a command over the existing TCP-IP connection. If your IMAP server supports that you don't have to manually check for new mail or have Thunderbird poll for new mail every x minutes.
- An IMAP account normally only downloads headers to the hard disk when checking for new mail. Your account would have an "Inbox.msf" file but no "Inbox" file. When you read a message it is fetched from the server unless its already cached in memory. If you want to maintain a local synched copy of a remote folder use "Tools -> Account Settings -> Offline & Disk space -> Offline -> Select folders for offline use". That will store/update a copy of the selected remote folder as an mbox file, which you can only see when working offline.
- IMAP provides several options for what Thunderbird should do when it deletes a message. Among other things, this can cause deleted messages to still be readable using either webmail or other email clients until you compact the folder. See the Deleting messages in IMAP accounts article for more information.
- Many IMAP servers default to a Sent Items folder, not a Sent folder. Thunderbird defaults to 'Sent' in "Tools -> Account Settings -> Copies & Folders -> When sending messages automatically". Thunderbird will not create the folder if it doesn't exist.
- Thunderbird has a small in-memory cache of recently opened IMAP messages. It is set to 4MB by default in Thunderbird version 3.1.x and earlier, and can be modified by a hidden preference. Some email clients support a local disk cache, and provide the ability to specify that any searches should search the disk cache first  . Thunderbird supports a disk cache since version 3.0, which can be modified in the advanced preferences.
- Thunderbird also supports downloading folders for offline use (synchronization), but that is different from the short-term cache. The synchronized copies are used when working offline using File -> Offline ->Work Offline, which that disables all network access. They are also needed for message-body search using the Global Search functionality , otherwise only the old search dialog and the quick-filter bar are available.