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.

• 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.

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