Importing and exporting your mail
From MozillaZine Knowledge Base
If your old email client is still installed, the easiest solution is to import the e-mail, address books and settings using Tools -> Import. If it is not installed or Thunderbird doesn't know how to import data from it, typically you need to find some way to export or convert your old email client's mail files to either .EML or mbox files, and some way to export or convert the address books to .CSV or .LDIF files. And if you just need to transfer email between email clients, another option is doing so indirectly via a mail server.
Thunderbird, Netscape, Mozilla Suite and SeaMonkey use mbox files to store the messages for a folder. The ImportExportTools extension can import and export mbox and .EML files. Many email clients use mbox files (regardless of whether they use no file extension, .mbx or .mbox as the file extension) to store messages or provide a way to export folders as mbox or .EML files due to the popularity of Eudora and Outlook Express. Sometimes email clients use "Unix format" to refer to a mbox file since the format was originally developed for Unix systems.
If you have problems importing .EML files using the ImportExportTools extension try using eml2mbx to convert the .EML files to a mbox file and then import the mbox file using the ImportExportTools extension. A Google search will find several eml to mbox conversion programs. Whats unusual about this one is that provides a lot of control over how it converts the .EML files using a "eml2mbx.ini" file.
If your old email client supports exporting the address book as either a .CSV or .LDIF file you can import it using Tools -> Import -> Address books -> Text Files. If not you might be able to use a program like Dawn to convert the address book to a .CSV file and then import it.
No matter how you import or export your e-mail its a good idea to back up your e-mail first in case something unexpected happens.
If you're moving from a old PC to a new one its typically easiest (and safest) to install Thunderbird on the old PC, import your e-mail, settings and address books and then move the Thunderbird profile to the new PC.
America Online (AOL)
Apple OS X Mail.app
Apple iPod (contacts and calendar only)
Barca and/or PocoMail
Courier (Rose City)
Eudora is no longer being sold by Qualcomm. They have created a open source version based on Thunderbird. See this article for more information.
Open Entourage and drag and drop the folders to the desktop. That will create mbox files with an .mbox file extension. Either drag and drop them to the appropriate directory in your Thunderbird profile or import them using the ImportExportTools extension.
How To Migrate From Evolution To Thunderbird In Ubuntu Intrepid describes how to migrate the mail, calender, tasks and address book. It was written when Evolution defaulted to using mbox files, just like Thunderbird does. Evolution 3.2.0 and later uses maildir files. If you created your Evolution profile using a version that defaulted to maildir you will need to use a tool to convert maildir files into mbox files for multiple folders, such as maildirarc or maildir2mbox. Then you can import the mbox files using the ImportExportTools add-on, and resume using the article for how to import your other data.
Groupwise supports SOAP, POP and IMAP accounts. The easiest way to migrate mail would be to drag and drop folders from a Groupwise IMAP account in Thunderbird to another IMAP account (such as Gmail) in Thunderbird. If thats not possible GWSave does a bulk export of messages to .eml files. You could use the ImportExportTools add-on to import them into Thunderbird.
Thunderbird supports using LDAP servers as address books so if your Groupwise server is configured to provide that, you can keep using it. Some other solutions:
Some other Groupwise related tools are here. Groupwise won't run unless its the default email client. If you try to have both Groupwise and Thunderbird co-exist, you'll need to uncheck Tools -> Options -> Advanced -> General -> "Always check to see if Thunderbrid is the default email client on startup" within Thunderbird.
Juno (the email client, not the ISP)
MH, mh-e, nmh, xmh, exmh
Mozilla Suite and Netscape 7
Outlook and Outlook Express
Polarbar Mailer / J Street Mailer
Webmail (including Google Gmail, MSN Hotmail, Netscape Mail, or Yahoo)
Windows Mail (aka Windows Live Mail)
Migrating messages using a mail server
Using an extension to put your Thunderbird messages back on the POP server
Another way to export your messages would be to follow the instructions in Putting messages back on a POP3 server to upload your messages back to your POP3 server. You could then download them normally with another email client. They will have normal headers, it would not look like you had forwarded the messages to yourself.
Using IMAP remote folders as an intermediary
If you have problems migrating your mail you might consider signing up for a free IMAP account and use it as a intermediary to move your mail. Think of it as a file share that knows about mail folders. You could create a 15GB IMAP account for Gmail using these instructions, a 5GB IMAP account with Outlook using these instructions or a 1TB IMAP account with Yahoo using these instructions . When you're done moving your mail you can delete the IMAP account in both email clients.
IMAP supports remote folders (on the mail server) that you can access just like they're local folders. You can copy/move messages/folders to/from them, even drag and drop a entire folder hierarchy. This would let you preserve most of the message status information (what messages have been replied to etc.) and the folder hierarchy. It should cause the least problems since you're using a Internet standard that both email clients support, rather than trying to convert between two partially documented file formats etc.
Create an IMAP account in both email clients. Copy your messages/folders to a remote inbox folder using one email client and then copy them from the remote inbox folder to a local folder using the other email client. If you have more messages than can fit in the mailbox do it in several steps, deleting the messages/folders in the remote inbox and compacting it as needed to free up more space. There are a few email clients whose IMAP support is crippled, and don't support uploading messages. But most popular email clients (Eudora, Outlook, Outlook Express, Pegasus, Thunderbird etc. ) do.
If the performance delay due to sending a lot of messages over the network is a issue you could temporarily install a local IMAP server and use it to provide the remote folders. hMailServer is an open source IMAP server for Windows thats easy to install.
Use a Gmail specific loader
The Gmail Loader reads messages from your existing mail files and forwards them to either the Inbox or Sent Mail folder in a Gmail account. It supports Mbox (what Mozilla uses), MailDir (Qmail, others), MMDF (Mutt), MH (NMH), and Babyl (Emacs RMAIL) formats. It hides the fact that the message is forwarded and eliminates the grunt work of individually forwarding all of your messages. GML has versions for Windows (all files needed are included) and Linux / OS X (requires you to install Python).
Google webMail Filer for Thunderbird is a utility to upload messages from Thunderbird's mail folders to your Gmail account. It deals with issues such as not uploading duplicates, Unicode, Gmail nested labels and the Google mail quota that its not clear that GML knows about. Its only available for Windows (its a .NET program that won't run under Wine or using Mono). Its a better way to upload mail to Gmail but since it is Thunderbird specific program that only knows about mbox files its not useful if you are trying to migrate messages from another email client. Version 18.104.22.168 (released 2013-01-29) is the last free version available. Supposedly all future development will be on a shareware version.
Google Apps Migration for Microsoft Outlook can be used to migrate messages, contacts and calendars from Outlook to a free Google Apps account. It replaces the Google Email Uploader program. You can create a POP or IMAP account for Google Apps in Thunderbird.
There are four main versions of the mbox format. Thunderbird, Mozilla Suite, Netscape, Portable Thunderbird, Postbox, Spicebird, SeaMonkey and the open source version of Eudora use a slightly modified version of the mboxrd variant of this format. The proprietary version of Eudora uses a slightly modified version of the mboxo variant. Most Windows email clients that use mbox files will use a mboxo or mboxrd version, and you should have little trouble importing them. Under Linux you're more likely to run into an mboxcl or mboxcl2 version, which is incompatible. Both Kmail and Evolution use a compatible version. You might try migrating your messages to Kmail as an intermediate step since it supports importing numerous formats.
See the following links for more technical information about this format:
Manually importing and exporting
Unless you are an advanced user its generally preferable to use an application to import or export a folder since that typically sanitizes the file to minimize any incompatibilities. If you use IMAP's optional offline mode to create a local copy of remote folders, think ahead about how you will deal with any potential synchronization issues. To manually import/export mbox formatted files:
Troubleshooting is beyond the scope of this article, but if you examine the mbox technical links and the links to importing specific programs, you may find helpful information.