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.

In this article

Background - The Internet is a dangerous neighborhood

How to share your files safely

Attachment file types blocked by Outlook

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



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