Emulate Microsoft email clients
From MozillaZine Knowledge Base
Thunderbird has roughly the same functionality as Outlook Express, but the look and feel is different. It doesn't have native support for Microsoft Exchange servers (though it can use the optional POP/IMAP/SMTP mail servers) so its not direct competition with Outlook. There are a couple of things you can do to make Thunderbird look and act more like Outlook and Outlook Express, though you should ask yourself why you switched if you want to mimic it too closely.
Thunderbird store address books in .MAB files. It can import address books from Microsoft email clients though it has different columns. Its possible to share Outlook and Outlook Express contacts though its not recommended .
The MoreFunctionsForAddressBook add-on adds several columns to the address book and lets you create categories. The Email crawler add-on adds addresses from the selected folder and its child folders to an address book.
Outlook uses .NK2 files to store email addresses for auto-completion. A proprietary tool is needed to edit or merge them. Thunderbird uses addresses from all of the address books for auto-completion. You can not specify which addresses get used for auto-completion - its all or none.
If an attachment contains any binary data (8bit bytes) it has to be encoded in order to send it as 7bit ASCII text. Thunderbird defaults to using Base64 encoding while Outlook defaults to Quoted Printable. If the file has few 8bit characters Quoted Printable is more efficient (creates a smaller attachment). However, if there are a lot of 8bit bytes in the file it is extremely inefficient. Which encoding scheme is better depends upon what type of attachments you typically send. You can switch to Quoted Printable by using the config editor to set mail.strictly_mime true, and then restart Thunderbird.
The Lightning extension provides integrated calendar support for Thunderbird.
The ReminderFox extension is an alternative to a full-fledged calendar system, focusing on providing date based reminders and to-do lists. It does not support sharing a calendar with Outlook/Exchange but can import a Outlook calendar if you use another program to export them as .ics files, and supports iCal Exchange and other online calendar services that support the the iCalendar (ics) format.
A few email clients such as Outlook may insert calendar events in the message body rather than as an attachment. If you import that message Thunderbird can't see the calendar event unless you use the Show hidden calendar events extension. 
Thunderbird has no concept of or equivalent to Outlook categories. Outlook stores the categories in a master category list (not in messages or folders), which Thunderbird is oblivious of.
Outlook can be configured to use Microsoft Word as the editor. Thunderbird doesn't let you replace the editor, though there is a External editor extension that makes it easier to compose the message using an external editor. The experimental add-on Compose for Thunderbird replaces the compose message window using the CKEditor WYSIWYG editor, and was meant to pave the way for supporting a pluggable editor.
The TitleCase add-on lets you select text and then press a shortcut to transform it into Title Case, Proper Case, Start Case, Camel Case, Upper Case, or Lower Case. There is a Auto Correct extension at http://dl.dropbox.com/u/11780544/TB_Auto_Correct_0.0.6.2.xpi  that auto-corrects words and automatically capitalizes the first word of a sentence. There is also the mms Auto Correct extension. Thunderbird has a built-in spelling checker. There is a Grammar Checker extension that detect errors that a simple spell checker cannot detect and also detects some grammar mistakes. The MailTweaks extension lets you move the splitter at the top of the message when composing a message to reduce the number of address lines and provides a permanent pen (like a pen in Outlook's ink tools) for adding comments in replies. However, that extension has been abandoned and most of its features don't work with versions greater than 3.1
Double clicking on a .eml file in Windows Explorer or opening it using File -> Open Saved Message only lets you view the message, you can't copy or move it anywhere. However, you can drag and drop it like in Outlook Express. The ImportExportTools extension supports importing and exporting .eml files.
Thunderbird doesn't have native support for the Microsoft Exchange server. It will work with the optional POP3, IMAP and SMTP servers if the admin installs them. It can access a Microsoft Exchange Global Address List (GAL) by treating it as a LDAP based address book.
If a driver (for example scanner software that emails your images) states it require MAPI support and doesn't mention what email clients it supports they probably mean Extended MAPI, which Thunderbird doesn't support. Thunderbird supports Simple MAPI but its buggy. However, this usually doesn't cause a problem with Microsoft applications using the default email client (Thunderbird) to send a message.
Thunderbird doesn't support Outlook Web Access (OWA) because it requires WebDAV support, and Thunderbird only supports fetching mail using POP3 and IMAP. This article mentions several extensions that provide OWA support but many users have problems getting them to work.
Thunderbird, Entourage, Outlook and Outlook Express support S/MIME.
Some versions of Outlook support Information Rights Management (IRM), which lets the sender specify who can forward, print, or copy and paste a message or set a e-mail expiration period. Outlook also supports a Federal Release Security Labels API which several add-ons use to support a security label policy. Thunderbird doesn't have an equivalent.   You also can't recall a message.
Outlook installed with the Corporate Workgroup/Other option can be used to vote (and process votes). Outlook uses extended MAPI calls to Exchange servers in the same workgroup to do that. Thunderbird can't do that since its basicly limited to what HTML provides.
Priorities are set using the X-Priority header, with 1 being the highest and 5 being the lowest priority. Thunderbird adds just X-Priority: 1 (Highest) while Outlook and Outlook Express adds X-Priority: 1 (Highest) , X-MSMail-Priority: High, and Importance: High headers.  Most email clients check the X-Priority header but somebody in a Microsoft Office-centric environment might use a message filter that checks one of the other headers. If this becomes a problem you could use the Mnenhy extension to make it easy to add custom headers.
Outlook supports RFC2047 for attachment filename handling while Thunderbird supports RFC2231, the newer standard. This can cause file attachments sent from Thunderbird to be renamed if they're longer than 64 bytes. See this article for how to prevent that.
Thunderbird doesn't support OneNote. There is a Firefox extension for OneNote but none for Thunderbird.
Look and feel
Thunderbird has multiple identity support, but its fundamentally different from what Outlook Express provides. Frequently this is an issue when somebody has secondary accounts and is used to using File -> Switch Identities to switch to a different account. See this article for how to use secondary accounts.
Outlook provides the ability to take notes. The QuickNote , Xnote and Notary extensions provide sticky notes, which are attached to a specific message. There is no equivalent of tossing notes into a folder or organizing them .
Thunderbird calls message rules message filters. They are not compatible, and Thunderbird doesn't know how to convert them from a Microsoft email client. There don't appear to be any utilities that know how to convert them either, possibly because there is no industry standard format for message rules/filters.
Outlook supports executing rules on messages before sending them (setting priority, requesting receipt, sending a copy of the message to certain people etc.). Thunderbird doesn't support that. The closest it has is a experimental Send Filter extension that applies rules to messages in the Sent folder (not to all outgoing messages before they're sent).
You can't configure Thunderbird to send messages in the background, the compose message window won't disappear until its finished sending. However, you can click on the main window and do something else (including composing another message) in parallel.
Thunderbird doesn't have a Send/Receive option, sending messages and checking for new ones are normally done independently. There are email scheduler plug-ins available for Outlook. See the Outbox article for several extensions that provide similar functionality.
Other users mailboxes
Outlook can be used to access other users mailboxes, if the Active Directory is setup to grant you access.
If you want to access other users mailboxes using an IMAP account you need to specify the mailbox location in Tools -> Account Settings -> Server settings -> Advanced -> Other users and be granted authorization to access the mailboxes. If the IMAP server isn't configured to grant you access based on Active Directory settings (it might not be that integrated with Exchange without some re-configuration by the admin) the admin would need to use something like ACL's to grant you access.
Frequently admins will enable the IMAP server to let you use a third party email client but aren't willing to do anything extra to duplicate Outlook specific functionality. However, OWA supports accessing other users mailboxes as long as you're granted full access to that mailbox. One solution might be to connect using DavMail. Its a a POP/IMAP/SMTP/Caldav/LDAP gateway allowing any e-mail/calendar client to access Exchange via OWA that you can install on your PC. See the Outlook Web Access article for more information.