Help, Outlook has blocked my attachments

March 11, 2010

in Exchange, Microsoft, Outlook

Most of us that have worked with Outlook for some time have come across this feature.  You try to send a file to a colleague and Outlook blocks and displays a nice message for you:

Outlook blocked access to the following potentially unsafe attachments: []

Of course the kneejerk reaction is to just rename the extension and resend the email.  While this works the majority of the time, it requires extra steps, time and some forethought before sending the email (while increasing your mailbox size).  A more permanent solution (albeit more dangerous) is to modify the Outlook policy itself either through a registry edit or using a GPO.

Other and more preferred options include:

  1. Use a file share or FTP site to save and access the attachment.  This is a Messaging administrator’s dream as it takes the burden off the Exchange servers and places it solely onto the file server.  While message storage is usually at a premium, file share storage can be much cheaper.
  2. Use a file compression utility to change the file name extension.  This works great and often saves valuable mailbox space in the process.  Two birds with one stone.

Searching the Internet on this topic brings up many posts (this is not an new topic), and while most seem to be focused solely on how to modify the behavior at the client level, I’ve included some links that the administrator can utilize to adjust the attachment filtering behavior at the server level.


For more information on Outlook’s attachment filtering behavior along with a table of all those extensions that Outlook (aka: Microsoft) deems unsafe following this link.

Outlook divides attachments into three groups based on their file name extension or file type.  Outlook handles each group in a specific way.

Level 1

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.

Level 2

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.


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.

Customizing Attachment Filtering

So now that you understand a bit more about why let’s focus on the how.  There are a couple of options for customizing attachment filtering behavior.  While this article’s language is specific to understanding and modifying the behavior of Outlook, I’ve also included some options for modifying the attachment filtering features at the server level.

Configure the behavior at the server level

Configure the behavior at the client level

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