Allowing blocked file types in Outlook

This guide was created for Windows XP users running Outlook 2007.

Users may receive a message in Outlook which states “Outlook blocked access to the following potentially unsafe attachments:”

This block is being enforced at the workstation level and means the file has successfully traversed the perimeter network firewall. The administrator is able to remove the block for specific filetypes if required. This fix persists after reboots and requires editing the registry.

To access the registry you can do a Start > Run > Regedit (or if you’re in a locked down environment you may need to either do a runas or browse to C:\Windows\System32, then right-click on Regedit.exe while holding shift and select “Run as” and authenticate with your admin account).

Once you have regedit open you need to open the HKEY_USERS key. The tricky bit now is working out which one of those SIDS is the logged in user.

You will have to do a bit of digging, but the key we are looking for is at

HKEY_USERS > *RANDOM SID THAT YOU HAVE TO WORK OUT* > Software > Microsoft > Office > 12.0 > Outlook > Security

(for Outlook 2003 users it will be Office > 11.0 and for Outlook 2010 users it will be Office > 14.0)

There should only be one SID which has a decent amount of information under it (it will be one of the longer ones and not S-1-5-20 or similar).

Once you have found the right SID and registry location do the following:

1. With the “Security” Key selected click Edit > New > String Value.

2. Type Level1Remove as the new value.

3. Modify the string Data to include the file types you wish to view in Outlook. For example: .exe

4. To specify multiple file types, use the following format: .exe;.com

5. Exit Registry Editor.

6. Restart Outlook.

Created with the assitance of Microsoft Support article at

Posted in Guides Tagged with:

Leave a Reply

Your email address will not be published. Required fields are marked *