From MozillaZine Knowledge Base
This article was written for Thunderbird but also applies to Mozilla Suite/SeaMonkey. The description of each program is deliberately limited to the most significant features to try to avoid being an advertisement. Only programs that are discussed in the forums are mentioned.
Aid4Mail is a Windows application to convert, export, migrate or archive messages. It's a shareware program with three different versions, and a trial mode. It supports more than 20 email clients and mailbox formats. Its more unusual features are that it can process folders on any drive (including a CD), that it preserves message status information such as "unread", "read", "replied", and "forwarded" when exporting, that it is able to strip HTML or embedded content, can export messages as MHT Web Archive or ZIP archive files, and that it tries to fix any MIME problems when exporting/importing.
Some of its versions support importing/exporting to Outlook using MAPI. However, it cannot access .MSG or .PST files without Outlook. The trial version is limited to processing a maximum of 50 messages per mail folder and exporting 10 messages per mail folder using the MHT format. The license is not tied to your PC.
Dawn is a freeware Windows application to convert an address book from one format to another. It supports Becky!, Eudora, Juno, Mozilla Suite, Netscape, Thunderbird, SeaMonkey, Outlook, Outlook Express, Opera, Pegasus, Palm, Pine, .LDIF, .CSV, and plain text address books.
DbxConv is a free Windows application to convert Outlook Express .DBX files to .mbox or .EML files. Thunderbird uses a mboxrd varation of a mbox file to store the messages for a folder so you'd probably want to use the -mbxrd option. You can use the ImportExportTools add-on to import the mbox or .EML files.
Dummy SMTP server
Sometimes for testing or troubleshooting it's useful to have a dummy SMTP server that doesn't actually send anything, but lets you see all of the communication between the email client and the SMTP server. DevNullSMTP does this. It's written in Java.
Emailchemy is a Java-based commercial program that runs under Windows, OSX and Linux and can convert messages into mbox, maildir, maildir++ , .CSV and .EML formats.
It can be used to export messages as either .eml or mbox files from a Outlook .PST file. You could then use the ImportExportTool add-on in Thunderbird to import the .eml or mbox files.
EnumProcess is a Windows utility that tries to list all firewall related processes. It is useful when you have a problem getting Thunderbird to make any connection, and you already tried everything you can think of to avoid your firewall from blocking Thunderbird.
Extra Folder Columns (+Expunged)
This add-on is a modified version of the extra folder columns add-on that adds a "expunged" column to the folder pane, which can be useful when troubleshooting why a folder is too big. It can be downloaded from this attachment in a bugzilla bug report. It appears to use data from each folder's index file (*.msf) and assumes that if you use a IMAP account that you never disabled the default settings to use a mbox file (just like POP accounts do). Its assumption are not a problem if you use a POP account or "Local Folders".
EudoraRescue , Eudora2Mbox and Eudora2unix are python scripts to sanitize Eudora folders and make them more mbox compatible, to eliminate problems when other email clients such as Thunderbird imports them. They were developed before Thunderbird 1.0, which seems to have solved most of the import problems but they might still be useful.
EZ ThunderBird Backup
EZ ThunderBird Backup is a commercial Windows program for backing up your messages and address book to a .ZIP file. It supports password protection and the ability to automate the backup using the the Windows Task Scheduler.
Upload mail to Gmail
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). However, if you are uploading messages from a mbox file you are better off using "Google webMail Filer for Thunderbird" instead.
Google webMail Filer for Thunderbird is a utility to upload messages from Thunderbird's mail folders to your Gmail account. Its actively maintained and 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.
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.
IMAPSize is a freeware Windows application to help maintain your IMAP account. It supports deleting and saving attachments, copying and moving messages from one account/server to another, message and mailbox management, modifying message headers, incremental backups, replicating your IMAP folder hierarchy to your local drive, download/upload messages to/from mbox and .EML files and both .EML to mbox and mbox to .EML conversions. It recognizes any messages marked by SpamAssassin as SPAM as SPAM, and can display the real size of your Inbox folder. The author plans on porting it to Linux.
IMAPSync is a Linux program to move messages between IMAP mailboxes. It reduces the amount of data transferred by only copying messages that are not in both mailboxes. You need to use it with a --subscribe command line argument in order for Thunderbird 3.x to be able to see/subscribe to any folders it creates. 
Jaback is a freeware backup utility that supports scheduling backup, copy, synchronize, and zip tasks. It can monitor a file or folder and back it up if it changes, and send a message to notify you if a task failed, based on the task exit code. Backups are standard ZIP files. There is both a Windows and a OSX version.
Mail Washer Free
The free version of MailWasher is an anti-spam and mail preview tool that you can use with Thunderbird to bounce messages. Bouncing spam messages doesn't help (the cost of a spam message is so ridiculously low they don't care how many go to invalid addresses) and you might actually be spamming somebody whose email address the spammer spoofed, but it can be useful if you need to bounce mail from a specific individual. The free version is limited to one POP account. See Download only certain POP messages for how to configure Thunderbird to preview messages instead of using a utility like Mail Washer.
MailStore Home is a free mail backup utility that supports both Thunderbird and Outlook. Its mainly intended as a way to backup and restore profiles but you can also use it to convert a .pst file into something that Thunderbird can import.
Select "Archive Email" and point it to a Outlook .PST file, export to "File System" using the .eml format, and then use the ImportExportTool add-on in Thunderbird to import the .eml files.
The best way to convert a mbox file (mail folder) to .eml files (one message per file) is to use the ImportExportTools add-on. It can also import .eml files into a mbox file. If you can't use that add-on for some reason the easiest solution (under Windows) is to use the mbox2eml utility. There is also a Java based equivalent (any operating system) and a C++ based equivalent (Windows, Linux) from different authors.
Mbox2XML is a free Windows application for backing up one or mbox files (folders) in a .XML file that can be displayed using a modern browser such as IE, Firefox or Opera. This converts any HTML messages to plain text. The user has the option to include attachments. It uses UTF-8 encoding, which provides support for most character sets.
MboxViewer is a free utility to view the contents of mbox files, with full MIME support, the ability to open mbox files larger than 4GB, and view attachments. It has a Windows executable and sources that are wxWidgets compatible. It is much easier to use than alternatives such as configuring a Windows version of Mutt  or Pine.
There don't appear to be any free mbox viewers for OS X or Linux. There is a MBox Office - Viewer and Migration Assistant sold in the Mac App Store. Archmbox is a Perl based script that lets you list, move, and copy messages from one mbox file to another. It does not support viewing them as its meant primarily for archiving messages. It supports Linux and OS X.
Mozbackup is a freeware backup program for Windows described in this article. It is the most popular way to back up Mozilla products under Windows.
Message Checker is a web based utility to check whether a message is well-formatted based on several standards, tradition, and the practices advocated by Dan's Mail Format Site. You need to either copy and paste the raw source of a message or upload a .EML file to it to have it analyze a message. View -> Message Source (or Control-U) will view a messages source. File -> Save As -> File will save a message as a .EML file.
Be careful what messages you validate.
Outlook2Mac is a commercial program to move Outlook email, contacts, and calendar appointments from a Windows PC to a Macintosh computer. It exports the Outlook data using .mbox, vCard, and iCal file formats.
Poptray is an open-source mail-checker for Windows. It supports rules that can play a sound, run another program, display a balloon popup, change the color of the icon in the system tray, delete the message from the server, mark the message as spam etc. The rules can include regular expressions. You can preview messages and/or delete messages without downloading them. It supports POP3 by default but has plugins for Hotmail, IMAP, Microsoft Speech, Flash the LEDs, SSL, YAC (Yet Another Caller ID Program) and Bayesian spam filtering. You have to look in its forums for some of the plugins.
The Wine Application Database states that PopTray runs "perfectly" under Linux although they didn't test any of its plug-ins. If you don't want to run it under Wine PopTrayMinus is a open source mail checker for Linux written by somebody else that tries to provides a useful subset of PopTray.
POP Peeper is yet another free mail-checker for Windows. It is less popular than PopTray. What's unusual about it is that it has a plugin that can repeat a sound until acknowledged and\or launch another program when new mail arrives, a skinnable popup when new email arrives, a screen-saver plugin and it can check POP, IMAP (has IDLE support) and webmail.
Free SMTP Server
Free SMTP Server is a small SMTP Server that can be installed under Windows. It's useful when you want to send a message directly to another PC on a LAN, you can't use your ISP's SMTP server because your notebook is connected to the Internet using a different ISP, or you want to work around some limitation in your email providers SMTP server.
An older version is available here. The reason for the link is that most download/freeware web sites download the software directly from Softstack's web site, and about a year Softstack stopped offering the freeware version.
blat, mailsend and sendEmail
Blat, mailsend and sendEmail are command line utilities that can be used to send messages via SMTP. They're not intended to be used as a SMTP server by Thunderbird. They (or Thunderbird or SeaMonkey) can also be used by the Send Page by Email add-on for Firefox to send the contents of a web page in the message body of a email.
XmlSendmail is a command line utility to send a message using SMTP. Its configured using a XML file that specifies the recipients, subject, the file containing the message body and any files that should be sent as attachments. It might be useful if you want to periodically compose a message with a editor, and then send it to the same recipients as last time.
Sift is a Linux utility that lets you run scripts against messages stored in a IMAP server. For example it could select all messages that meet a certain criteria, and then pass the message body of each of those messages (one by one) to another program.
SysTools MBOX Convertor
SysTools MBOX Convertor is a commercial Windows application to convert multiple mbox files into .PST, .EML or .MSG files. Few convertors support converting mbox files to .MSG files without requiring an existing Outlook installation. They also sell a SysTools Outlook to MBOX program.
ThunderStor is a shareware Windows application to export messages and address books from Thunderbird. The writeup is confusing as it appears to claim it can export messages as .EML, .MSG and .TXT files but it also describes the -.msg command line option as "Forces output files to the extension of .msg". Forcing a file extension is not the same thing as converting to that file format, but this might just be poor wording. It can also extract or merge attachments and recover deleted messages. It requires the Microsoft .NET framework. The trial version will process up to 250 messages.
Unlocker is a Windows utility that lists locks on a file and lets you remove them. While its not a mail utility, its useful if something locks a file in your profile. For example, if you get a "Unable to load address book file abook.mab. It may be read-only, or locked by another application. Please try again later." error message.
A similar program for OS X is LockUnMatic.