I found that when I created a custom outlook add-in to run on the Send event, that it doesn't run when I deploy it on another machine (works fine on my own). After much research, I found that a requirement on the other machine is not included in the deployment files. We needed Extensibility.dll to be installed on the target machine. For some reason, this is not included in VS 2008, despite there being a KB article for this file for VS2005. Research shows that the majority of people had to downgrade back to VS2005 to get Extensibility.dll installed on target machines, but that is not necessary.
First you need to have Visual Studio 2005 installed somewhere so you can download the kb article from here and install it: http://support.microsoft.com/kb/908002. Without VS 2005, you won't be able to install this kb (which I find silly because you need it for VS2008).
After installing, you can then do one of the following 2 ways.
The hard way...
1. Navigate to C:\Program Files\Microsoft Visual Studio 8\SDK\v2.0\BootStrapper\Packages, locate KB908002 folder.
2. Log on to the machine which has Visual Studio 2008 installed and copy KB908002 folder to C:\Program Files\Microsoft SDKs\Windows\v6.0A\Bootstrapper\Packages
Now if you go to the pre-requisites list on VS 2008 (right click on the Setup and view properties, then click prerequisites), you'll see a new prerequisite, Shared Add-in Support Update for Microsoft .NET Framework 2.0. Check that and select "Download prerequisites from the same location as my application. This way, when you build it, a new folder is created with the required MSI in your deployment folder.
Or the easier way...
Navigate to C:\Program Files\Microsoft Visual Studio 8\SDK\v2.0\BootStrapper\Packages on the PC with VS2005 on it, locate KB908002 folder. Copy the extensibilityMSM.msi file to the target machine and install it there.
Which method you choose will depend on your own personal requirements.
Note that depending on what you need, you may need to right click on "Office.dll" in the Detected Dependencies folder and include it. I had to include Office.dll in my project too for the add-in to function.