In Microsoft Outlook, when receiving an e-mail that contains an attachment, you receive this alert message on top of the message or in the Reading Pane:
Outlook blocked access to the following potentially unsafe attachments: 
All versions of Outlook since Outlook 2000 Service Release 1 (SR1) include a security feature that blocks attachments posing a risk of viruses or other threats. Outlook blocks access to the attachment, but the attachment is present in the e-mail message.
This article describes how to open a blocked attachment, and what types of attachments Outlook may block.
When Outlook blocks an attachment, you can't save, delete, open, print, or otherwise work with the attachment in Outlook. Use one of the methods described below to safely access the attachment:
The first four methods are designed for beginer to intermediate computer users. If these methods don't work for you and if you're comfortable with advanced troubleshooting, use the methods described in the "Advanced troubleshooting" section.
Some methods require to restart your computer. You may want to print this article first if you choose one of these methods.
Method 1: Use a file share to access the attachment
You might want to ask the sender to save the attachment to a server or an FTP site that you can access. Ask the sender to send you a link to the attachment on the server or FTP site. You can click the link to access the attachment and save it on your computer.
If you need help using the server or FTP site, you can ask the sender for help, or you can contact the server administrator for more information.
Method 2: Use a file compression utility to change the file name extension
If no server or FTP site is available to you, you can ask the sender to use a file compression utility, such as WinZip, to compress the file. This creates a compressed archive file that has a different file name extension. Outlook does not recognize these file name extensions as potential threats. Therefore, it does not block the new attachment.
When the sender resends the new attachment to you, you can save it on your computer, and then you can use the third-party file compression software to extract the attachment. If you need help using the third-party file compression software, see your product documentation.
Method 3: Rename the file to have a different file name extension
If third-party file compression software is not available to you, you might want to request that the sender rename the attachment to use a file name extension that Outlook does not recognize as a threat. For example, an executable file that has the file name extension .exe could be renamed as a Word 97 file that has a .doc file name extension.
Ask the sender to resend the renamed attachment to you. After you receive the renamed attachment, you can save it to your computer and rename the file again to use the original file name extension.
Follow these steps to save the attachment and rename it to use the original file name extension:
Locate the attachment in the e-mail.
Right-click the attachment, and then click Copy.
Right-click the desktop, and then click Paste.
Right-click the pasted file, and then click Rename.
Rename the file to use the original file name extension, such as .exe.
Method 4: Ask the Exchange server administrator to change the security settings
If you use Outlook with a Microsoft Exchange server and the administrator has configured the Outlook security settings, the administrator might be able to help you. Ask the administrator to adjust the security settings on your mailbox to accept attachments such as the one that Outlook blocked.
If these methods did not work for you, and you are comfortable with advanced troubleshooting, please try the steps in the "Advanced troubleshooting" section.
If you are not comfortable with advanced troubleshooting, unfortunately this content is unable to help you any more. For your next steps, you might want to ask someone for help, or you might want to contact Support. For information about how to contact Support, please visit the following Microsoft Web site:
If you do not use Outlook with an Exchange server or if the Exchange server administrator lets users change the Outlook attachment security behavior, use method 1: "Customize attachment security behavior."
If you use Outlook with an Exchange server and the Exchange Server administrator has disallowed changes to the Outlook attachment security behavior, use method 2: "Configure Outlook in an Exchange environment."
Method 1: Customize attachment security behavior
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows
Important Before you can customize the attachment security behavior in Outlook 2000 SR1 and Microsoft Outlook 2000 SR1a, you must first apply either Microsoft Office 2000 Service Pack 2 or Microsoft Office 2000 Service Pack 3.
Follow these steps to modify the registry and change Outlook's attachment security behavior.
Exit Outlook if it is running.
Click Start, and then click Run. Copy and paste (or type) the following command in the Open box, and then press ENTER:
Verify that the following registry key for your version of Outlook exists.
Microsoft Office Outlook 2016
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Security Microsoft Office Outlook 2013
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Security Microsoft Office Outlook 2010
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Security Microsoft Office Outlook 2007
HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Security Microsoft Office Outlook 2003
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Security Microsoft Outlook 2002
HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security Microsoft Outlook 2000
HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Outlook\Security If the registry key exists, go to step 5.
If the registry key does not exist, follow these steps to create it:
Locate, and then click the following registry key:
Under Edit, click New, and then click Key.
Type Office, and then press ENTER.
Under Edit, click New, and then click Key.
For Outlook 2016, type 16.0, and then press ENTER.
For Outlook 2013, type 15.0, and then press ENTER.
For Outlook 2010, type 14.0, and then press ENTER.
For Outlook 2007, type 12.0, and then press ENTER.
For Outlook 2003, type 11.0, and then press ENTER.
For Outlook 2002, type 10.0, and then press ENTER.
For Outlook 2000, type 9.0, and then press ENTER.
Under Edit, click New, and then click Key.
Type Outlook, and then press ENTER.
Under Edit, click New, and then click Key.
Type Security, and then press ENTER.
Under Edit, click New, and then click String Value.
Copy and paste (or type) the following name for the new value:
Right-click the new string value name, and then click Modify.
Type the file name extension of the file type that you want to open in Outlook. For example:
.exe To specify multiple file types, use the following format:
Note You must specify the extension without the dot/period such as exe or exe;com in Microsoft 365 versions of Outlook prior to 16.0.12923.10000. From that version on, Outlook supports file types with or without the dot/period in the extension.
Exit Registry Editor.
Restart your computer.
When you start Outlook, open the file types you specified in the registry.
Note We recommend that you enable only the file types that you have to have. If you rarely receive a particular file type, we recommend that you give Outlook temporary access to the file type that is in question. Then, reconfigure Outlook to block the file type by undoing the changes to the registry. For more information about how you can configure Outlook to block attachment file name extensions that Outlook does not block by default, click the following article number to view the article in the Microsoft Knowledge Base:
837388 How to configure Outlook to block additional attachment file name extensions
Method 2: Configure Outlook in an Exchange environment
If you run Outlook in an Exchange environment, the Exchange server administrator can change the default attachment security behavior. For more information about how to configure Outlook in an Exchange environment, click the following article numbers to view the articles in the Microsoft Knowledge Base:
290499 Administrator information about e-mail security features
263297 Administrator information about the Outlook E-mail Security update: June 7, 2000
Attachments are divided into three groups based on their file name extension or file type. Outlook handles each group in a specific way.
Level 1 (Unsafe)
The unsafe category represents any file name extension that may have script or code associated with it. You cannot open any attachment that has an unsafe file name extension. For a list of the unsafe file name extensions, see this Office support page. The following list describes how Outlook behaves when you receive or send an unsafe file attachment:
You cannot save, delete, open, print, or otherwise work with unsafe files. A message at the top of the e-mail message indicates that Outlook has blocked access to the unsafe attachment. The attachment is inaccessible from Outlook. However, the attachment is not actually removed from the e-mail message.
If you forward an e-mail message that has an unsafe attachment, the attachment is not included in the forwarded e-mail message.
If you send an e-mail message that contains an unsafe attachment, you receive a warning message that states that other Outlook recipients may be unable to access the attachment that you are trying to send. You can safely ignore the warning message and send the e-mail message, or you can decide not to send the e-mail message.
In Outlook 2003, if you save or close an e-mail message that contains an unsafe attachment, you receive a warning message that states that you will be unable to open the attachment. You can override the warning message and save the e-mail message.
You cannot use the Insert Object command to open objects that are inserted in Microsoft Outlook Rich Text e-mail messages. You see a visual representation of the object. However, you cannot open or enable the object in the e-mail message.
You cannot open unsafe files that are stored in an Outlook or an Exchange folder. Although these files are not attached to an Outlook item, they are still considered unsafe. When you try to open the unsafe file, you receive the following error message:
Can't open the item. Outlook blocked access to this potentially unsafe item.
Level 2 files are not unsafe. However, they do require more security than other attachments. When you receive a Level 2 attachment, Outlook prompts you to save the attachment to a disk. You cannot open the attachment in the e-mail message. By default, file name extensions are not associated with this group. However, if you use Outlook with an Exchange server and your mail is delivered to an Exchange mailbox, the Exchange server administrator can add file name extensions to the Level 2 list.
When you try to open an attachment that has a file name extension other than those in the Level 1 or the Level 2 list, Outlook prompts you to either open the file directly or save it to a disk. You can turn off future prompts for that file name extension if you clear the Always ask before opening this type of file check box.
Note If a program associates itself with a new file name extension, Outlook treats that file name extension as safe until you add the file name extension to the list of Level 1 or Level 2 file name extensions.
For example, if you install a program on your computer that uses files that have a .xyz file name extension, when you open an attachment that has a .xyz file name extension, the program opens and runs the attachment. By default, the .xyz file name extension does not appear on the Level 1 or the Level 2 list. Therefore, Outlook treats it as a safe file name extension. If you want Outlook to treat attachments that have the .xyz file name extension as unsafe, you must add the .xyz file name extension to the list of Level 1 file name extensions.
For more information, read article 284414 in the Microsoft Knowledge Base:
The recipient receives an "Outlook blocked access to the following potentially unsafe attachments" error message when you send an e-mail message that contains a shortcut to a file in Outlook
For more information about blocked attachments in Outlook, visit this Microsoft Office Online Web site:
926512 Information for administrators about e-mail security settings in Outlook 2007