Earlier today I sent a Microsoft Access database to a colleague, and he could not access it. The problem is that Outlook does not by default allow you to even save an access database that has been sent… The rationale is that these documents can contain viruses or other harmful code, but the fact that Outlook doesn’t let you disable or at least customize this feature is just plain stupid. Here’s how to do it:

To solve this, open the Registry Editor (Regedit), and Expand the branches to HKEY_CURRENT_USER Software Microsoft Office 10.0 Outlook Security
Create a new string value by selecting New and then String Value from the Edit menu. Name the new value Level1Remove. Double-click the new Level1Remove to edit it, and enter the filename extensions you’d like to stop Outlook from blocking. Extensions should be typed in lower case, without the dots (.), and separated by semicolons (;). For example, type exe;mdb;vbs to allow .exe, .mdb, and .vbs attachments, respectfully.
Then Click Ok and then close the Registry Editor when you’re done. You may have to restart Outlook for this change to take effect.

So simple, but poorly documented…