Some file extensions are blocked by Microsoft Outlook for the potential damage that they may cause. File types blocked include EXE, COM, MDB and many others.

Outlook displays a message that it has blocked the attachment:

Sometimes it becomes necessary to “unblock” a particular file extension. One of the most common requests is to unblock Access database files (*.mdb). Let’s see how this can be done:

1. On the desktop running Outlook, open Registry Editor.
2. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\xx.x \Outlook\Security where xx.x is your Outlook version number (9.0, 10.0, or 11.0)
3. Add a new string named Level1Remove
4. Add value to this string with all the extensions that you want to unblock, separated by a semicolon. (For example: .mdb;.url ) Remember to put the dot before the extension.

It should, however, be kept in mind that unblocking a particular file type introduces new risk, as the user can also receive a malicious file of the same type from another user or the internet and he/she might inadvertently open it.

For Outlook 2007, you need to insert the string in the following key HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\12.0\Outlook\Security. If the key path doesn’t exist, you can create it.