Outlook Express

This article is about the discontinued Windows component. For the personal information manager included in the Microsoft Office suite, see Microsoft Outlook.
Outlook Express
A component of Microsoft Windows

Screenshot of Outlook Express in Windows XP
Type Email client, News client
Included with Windows 98, Windows Me, Windows 2000, Windows XP, Windows Server 2003
Also available for Windows 3.x, Windows NT 3.51, Windows 95, Windows NT 4.0,
Mac System 7, Mac OS 8, Mac OS 9
Replaced by Windows Mail,
macOS Mail
Related components
Internet Explorer

Outlook Express, formerly known as Microsoft Internet Mail and News, is a discontinued email and news client included with Internet Explorer versions 3.0 through 6.0. As such, it was bundled with several versions of Microsoft Windows, from Windows 98 to Windows Server 2003, and was available for Windows 3.x, Windows NT 3.51, Windows 95, Mac System 7, Mac OS 8, and Mac OS 9. In Windows Vista, Outlook Express was superseded by Windows Mail. In macOS, Outlook Express was superseded by Apple Mail.

Outlook Express is a different application from Microsoft Outlook. The two apps do not share a common codebase, but they do share a common architectural philosophy.[1] The similar names lead many people to conclude incorrectly that Outlook Express is a stripped-down version of Microsoft Outlook. Outlook Express uses the Windows Address Book to store contact information and integrates tightly with it. On Windows XP, it also integrates with Windows Messenger.


Microsoft Internet Mail and News

Version 1.0 was released as Microsoft Internet Mail and News in 1996 following the Internet Explorer 3 release.[2][3] This add-on precedes the Internet Mail profile for Microsoft Exchange 4.0 bundled in Windows 95. Version 2.0 was released at the end of 1996. Internet Mail and News handled only plain text and rich text (RTF) email, lacking HTML email.

In 1997 the app was changed and renamed as Outlook Express and bundled with Internet Explorer 4. The executable file for Outlook Express, msimn.exe, is a holdover from the Internet Mail and News era. Like Internet Explorer, Outlook Express 4 can run on Mac System 7, OS 8, and OS 9.

At one point, in a later beta version of Outlook Express 5, Outlook Express contained a sophisticated and adaptive spam filtering system; however this feature was removed shortly before launch. It was speculated on various websites and newsgroups at that time, that the feature was not stable enough for the mass market. Nearly two years later, a similar system, using a similar method of adaptive filtering, appeared as a feature of Microsoft Outlook.

Internet Explorer 5 required Outlook Express 5 to save Web Archive files (see MHTML).[4]

Outlook Express 6 is the last version of the software with Outlook branding. It has a similar layout to Outlook Express 5. It was included with Windows XP.

Versions for Windows

See also: Windows Mail

Versions for Mac


Beta screenshots of Outlook Express 7 were spotted in early builds of Windows Vista.[5] and relied on WinFS for the management and storage of contacts and other data.[6] In October 2005, Microsoft announced that Vista would exclusively include a new application named Windows Mail, based on large parts of Outlook Express source code.[7] In November 2007, Microsoft released Windows Live Mail (originally Windows Live Mail Desktop), a mail client based on Windows Mail and Outlook Express' source code, as a downloadable alternative for Outlook Express on Windows XP.

Outlook Express cannot run on macOS.[8] A built-in mail app was publicly introduced in the initial release of macOS, called Mac OS X Cheetah. Microsoft Entourage (up to the 2008 edition) and then Microsoft Outlook (in Office 2011 and later), sold as part of Microsoft Office for Macintosh, can be considered an indirect paid replacement for Outlook Express.


Email standards

Beginning with the text-based Unix Mail command, email traditionally used the inline or bottom posting styles when replying to messages. Outlook Express, as well as Lotus Notes and Microsoft Outlook, top-post (show replies newest to oldest) by default.

Microsoft Outlook and Microsoft Exchange use a proprietary email attachment format called Transport Neutral Encapsulation Format (TNEF) to handle formatting and other features specific to Outlook such as meeting requests. Outlook Express and other email clients are unable to read this format.[9] This can be confusing to Outlook Express users (as well as those who use other email clients) who receive attachments sent from Outlook.

Database corruption

Outlook Express has been prone to a number of problems which can corrupt its files database, especially when the database increases in size due to an increasing number of stored emails and during database compaction. This has led to a thriving market for apps that can back up, restore and recover corrupted files.[10] An open-source project called UnDBX was also created, which seems to be successful in recovering corrupt databases.[11] Microsoft has also released documentation which may be able to correct some non-severe problems and restore access to email messages, without resorting to third-party solutions.[12]

However, with the latest updates applied, Outlook Express now makes backup copies of DBX files prior to compaction. They are stored in the Recycle Bin. If an error occurs during compaction and messages are lost, the DBX files can be copied from the recycle bin.

Security issues

Outlook Express was one of the earlier email clients to support HTML email and scripts. As a result, emails were commonly infected with viruses.[13][14] Previously, another security flaw was that a script could automatically be opened as an attachment. Another bug was in Outlook Express's attachment handling that allowed an executable to appear to be a harmless attachment such as a graphics file. Opening or previewing the email could cause code to run without the user's knowledge or consent. Outlook Express uses Internet Explorer to render HTML email. Internet Explorer has been widely criticized for having numerous security flaws.

With Outlook Express SP2 (part of Windows XP SP2), Microsoft has tried to correct the security holes. Outlook Express now blocks images inside emails by default.[15][16] It uses only the restricted security zone for HTML email, which disables scripts and imposes restrictions on what web content can be rendered. It also warns when opening potentially malicious attachments.[17]

Handling of PGP/MIME signed messages

Outlook Express does not correctly handle MIME,[18] and will not display the body of signed messages inline. Users get a filled email and one attachment (one of the message text and one of the signature) and therefore need to open an attachment to see the email. If the email has been forwarded several times, users need to open attached email messages one inside the other multiple times to reach the parent email message. This bug has still not been rectified. The proper behavior is described in RFC 1847. When replying or forwarding a message to a user who has a digital signature, Outlook Express gives an error and does not allow the user to continue if there is no digital signature installed for the sender.[19]


Outlook Express does not have a documented object model like Microsoft Outlook. Programmatic access to, or control of Outlook Express for custom messaging applications or plugins is not officially documented and supported by Microsoft.[20] IStoreNamespace and IStoreFolder interfaces[21] were documented in 2003, but they are only related to the storage.

Some companies have managed to create their plugins hacking the interface using Windows hooks. A significant one was the PGP plugin because it was the only example of a working plugin whose source code was available (licensed under GPL). By 2003, some companies provided commercial solutions to develop add-ins.[22]

Office 2007 spell checker incompatibility

Outlook Express does not have a dedicated spell checker. It can use the spell checkers from Microsoft Office if Office is also installed. However, the Office 2007 spell checkers, except for the French spell checker, are incompatible with Outlook Express. Microsoft has acknowledged this problem,[23] but does not provide any sort of remedy. A solution is to install any pre-Office 2007 proofing tools or use a third party spell-checking app for Outlook Express. Windows Live Mail, which is based on Windows Mail's source code, contains built-in spell checking support (now for US English and numerous other languages) and is freely downloadable for Windows XP.

Glitches and other unusual aspects

Cancelling sending an email while it is being sent does not effectively prevent it from being sent.[24] Similarly, when importing .PST files, cancelling the import while it is in progress merely cancels the import of the current folder and the import resumes with the next folder. Furthermore, Outlook Express only supports .dbx files that are smaller than 2 GB, and may have performance issues when dealing with files approaching that limit.[25]

See also


  1. https://support.microsoft.com/en-us/kb/2419492
  2. Scott Schnoll. "The History of Internet Explorer". Archived from the original on 16 August 2011.
  3. Nwnetworks.com The History of Microsoft Internet Explorer
  4. "Microsoft.com<!- Bot generated title -> new Features in Internet Explorer 5". Support.microsoft.com. 2007-10-23. Retrieved 2012-12-17.
  5. Thurrott, Paul. "Windows Longhorn Build 4051 Gallery 3". Supersite for Windows. Retrieved February 9, 2015.
  6. Jennings, Roger (February 1, 2004). "Get a Grip on Longhorn". MVP Magazine. Retrieved February 9, 2015.
  7. Winsupersite.com, Windows Live Hotmail Review, Part 2 Windows Live Mail and Outlook Connector Archived February 17, 2008, at the Wayback Machine.
  8. https://discussions.apple.com/thread/136991?start=0&tstart=0
  9. "How email message formats affect Internet email messages in Outlook". Support.microsoft.com. 2011-10-03. Retrieved 2012-12-17.
  10. Koch, Tom (18 November 2004). "The Other E-Mail Threat: File Corruption in Outlook Express". microsoft.com. Microsoft. Archived from the original on 2 January 2008.
  11. UnDBX, sourceforge.net
  12. Koch, Tom (24 May 2005). "An Outlook Express basic repair kit". microsoft.com. Microsoft. Archived from the original on 8 August 2005.
  13. https://www.cfesa.com/Membersonly/security/email.htm
  14. http://www.techimo.com/forum/technical-support/59062-virus-problems-outlook-express.html
  15. ""Cannot Open email Attachments in Outlook Express After You Install SP1" August 15, 2007 Microsoft Support:". Support.microsoft.com. 2011-09-28. Retrieved 2012-12-17.
  16. Microsoft Support: "Images are blocked when you open an email message in Outlook Express on a Windows XP Service Pack 2-based computer", April 16, 2007
  17. ""Description of how the Attachment Manager works in Windows XP Service Pack 2", July 6, 2006 Microsoft Support". Support.microsoft.com. 2011-04-25. Retrieved 2012-12-17.
  18. "GPG Signing: Traditional vs. PGP/Mime". Phildev.net. Retrieved 2012-12-17.
  19. "You Receive an Error Message When You Try to Forward or to Reply to a Digitally Signed E-Mail Message". Microsoft Support (2.0 ed.). Microsoft. 8 December 2015.
  20. "Developer Support Limitations with Outlook Express". Support.microsoft.com. 2005-06-25. Retrieved 2012-12-17.
  21. "IStoreNamespace Interface". Msdn2.microsoft.com. 2011-06-30. Retrieved 2012-12-17.
  22. "Outlook Express API". Nektra.com. Retrieved 2012-12-17.
  23. "You no longer have spell checking capabilities in some languages in Outlook Express 6.0 after you install the 2007 Microsoft Office system". Support.microsoft.com. 2011-09-20. Retrieved 2012-12-17.
  24. "Outlook - Recalling a Message UCSF". Oaais.ucsf.edu. 2012-08-09. Retrieved 2012-12-17.
  25. "Information about the maximum file size of the .dbx files that are used by Outlook Express". Microsoft Support. Microsoft. Retrieved 7 February 2014.
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