Blocked attachments in Outlook

To help protect your computer, Microsoft Outlook does not allow you to receive files of certain types (such as .exe files) as attachments, because of their potential for introducing a virus into your computer. Outlook blocks these files by default. If your organization uses Microsoft Exchange, only the e-mail server administrator can unblock these file types. End users of Outlook cannot unblock these file types.

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Background - The Internet is a dangerous neighborhood

When Microsoft Outlook 97 first arrived on the market, the Internet still seemed like a relatively safe place, and sending attachments in e-mail seemed like a great idea. But then — beginning in March of 1999 when the Melissa virus struck, only to be followed by the ILOVEYOU virus — reality hit hard. These were the fastest-spreading viruses ever seen up to that time, and they forced a number of companies to shut down their e-mail systems, at significant cost. In order to spread, these attacks relied heavily on fooling people into believing that infected e-mail messages were coming from someone familiar, and even many computer experts fell victim to them and became infected. It is no secret that hackers like to attack the largest available target — and today, that is Microsoft Office Outlook. World wide, more than 300 million people use Outlook. Therefore, the industry and Microsoft customers look to Microsoft to provide protection against security threats.

Most people agree that the Internet is a wonderful tool, but it is important to remember that it is also a dangerous neighborhood. Even people who "know what they are doing" are vulnerable. And hackers are getting more sophisticated. Microsoft does not want Outlook to be a medium by which dangerous files can be spread around the Internet. Outlook, therefore, blocks potentially dangerous attachments and prevents unauthorized programs from accessing your address book. Outlook is designed for safety first, so that everyone can keep sending and receiving e-mail.

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How to share your files safely

The vast majority of people never intend to circulate potentially dangerous files — and the people who do intend to, use other methods. Here are a few safe ways to share files:

  • Outlook does not block documents that have the following file name extensions: .xls, .doc, .ppt, and .txt. These file types are the ones that most people use in their work. However, it is important to keep in mind that such files can contain macros that have the potential to spread viruses. So whenever such files arrive in your Inbox, use a third-party antivirus program to scan them before you open them. Also, follow the recommended directions to protect your PC.
  • To send a file of a blocked type, you can change the file name extension so that it does not match any of the blocked ones. For example, you can rename program.exe as program.exe_ok, and then attach the file to your e-mail message. Include instructions in the message about how the recipient can rename the file by using the original extension (in this case, as program.exe). The key point to remember here is that you should only open files that you expect from someone.
  • Use a zip program to package files before you attach them to your e-mail message. In the message, you can include instructions that explain how to extract the files from the zipped package.
  • Post the files to a secure network share. Most Internet service providers (ISPs) offer paying subscribers a space where they can post files. One method is to save files on MSN and give your recipients access to them. In your message to the recipients, you can include a link to the share.

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Attachment file types blocked by Outlook

The following table identifies the file types that Outlook blocks by default.

File name extension File type
.ade Access Project Extension (Microsoft)
.adp Access Project (Microsoft)
.app Executable Application
.asp Active Server Page
.bas BASIC Source Code
.bat Batch Processing
.cer Internet Security Certificate File
.chm Compiled HTML Help
.cmd DOS CP/M Command File, Command File for Windows NT
.cnt Help file index
.com Command
.cpl Windows Control Panel Extension (Microsoft)
.crt Certificate File
.csh csh Script
.der DER Encoded X509 Certificate File
.exe Executable File
.fxp FoxPro Compiled Source (Microsoft)
.gadget Windows Vista gadget
.hlp Windows Help File
.hpj Project file used to create Windows Help File
.hta Hypertext Application
.inf Information or Setup File
.ins IIS Internet Communications Settings (Microsoft)
.isp IIS Internet Service Provider Settings (Microsoft)
.its Internet Document Set, Internet Translation
.js JavaScript Source Code
.jse JScript Encoded Script File
.ksh UNIX Shell Script
.lnk Windows Shortcut File
.mad Access Module Shortcut (Microsoft)
.maf Access (Microsoft)
.mag Access Diagram Shortcut (Microsoft)
.mam Access Macro Shortcut (Microsoft)
.maq Access Query Shortcut (Microsoft)
.mar Access Report Shortcut (Microsoft)
.mas Access Stored Procedures (Microsoft)
.mat Access Table Shortcut (Microsoft)
.mau Media Attachment Unit
.mav Access View Shortcut (Microsoft)
.maw Access Data Access Page (Microsoft)
.mda Access Add-in (Microsoft), MDA Access 2 Workgroup (Microsoft)
.mdb Access Application (Microsoft), MDB Access Database (Microsoft)
.mde Access MDE Database File (Microsoft)
.mdt Access Add-in Data (Microsoft)
.mdw Access Workgroup Information (Microsoft)
.mdz Access Wizard Template (Microsoft)
.msc Microsoft Management Console Snap-in Control File (Microsoft)
.msh Microsoft Shell
.msh1 Microsoft Shell
.msh2 Microsoft Shell
.mshxml Microsoft Shell
.msh1xml Microsoft Shell
.msh2xml Microsoft Shell
.msi Windows Installer File (Microsoft)
.msp Windows Installer Update
.mst Windows SDK Setup Transform Script
.ops Office Profile Settings File
.osd Application virtualized with Microsoft SoftGrid Sequencer
.pcd Visual Test (Microsoft)
.pif Windows Program Information File (Microsoft)
.plg Developer Studio Build Log
.prf Windows System File
.prg Program File
.pst MS Exchange Address Book File, Outlook Personal Folder File (Microsoft)
.reg Registration Information/Key for W95/98, Registry Data File
.scf Windows Explorer Command
.scr Windows Screen Saver
.sct Windows Script Component, Foxpro Screen (Microsoft)
.shb Windows Shortcut into a Document
.shs Shell Scrap Object File
.ps1 Windows PowerShell
.ps1xml Windows PowerShell
.ps2 Windows PowerShell
.ps2xml Windows PowerShell
.psc1 Windows PowerShell
.psc2 Windows PowerShell
.tmp Temporary File/Folder
.url Internet Location
.vb VBScript File or Any VisualBasic Source
.vbe VBScript Encoded Script File
.vbp Visual Basic project file
.vbs VBScript Script File, Visual Basic for Applications Script
.vsmacros Visual Studio .NET Binary-based Macro Project (Microsoft)
.vsw Visio Workspace File (Microsoft)
.ws Windows Script File
.wsc Windows Script Component
.wsf Windows Script File
.wsh Windows Script Host Settings File
.xnk Exchange Public Folder Shortcut

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Applies to:
Outlook 2007