+2  A: 

Sounds like you might be better off creating a custom MAPI Service Provider, and then connecting Outlook to it. That should give you a (relatively) clean programming model to follow without having to worry about messing with the Outlook UI... and also help avoid problems caused by users accidentally clicking the wrong button!

Shog9
+3  A: 
A. Levy
That would be so rare as to be almost unheard-of. Probably no more common than people clicking "reply-to-all" when they mean "reply". :)
Aric TenEyck
I agree that there are issues, A. Levy. At this point I'm just trying to figure out what's possible and at how easily.
Zack Peterson
Forget about clicking - the shortcut key for Send is something like Ctrl+Enter; unless you remap that as well, you'll end up with folks triggering it out of habit. And if you DO remap it, you'll frustrate those same people when they go to use their normal email...!
Shog9
+2  A: 
76mel
+3  A: 

You absolutely can. You can use VSTO to do it. Here is an example of adding a context menu item to folders, but adding a button is very similar exercise.

JP Alioto
adding a button to a toolbar/ribbon might be similarly easy but I have some difficulty believing that inserting a button right between the built-in controls is as well - then again, I haven't ever used VSTO to build addins so far...
Oliver Giesen
+2  A: 

A simpler way could be, sending the message which contains the link to the secured content on your website.

This will avoid the need of creating an addin/accidentally hitting "send" instead of "send secure".

EDIT: I mean, even if you send the email by accident, there won't be anything in it other than some instructions with a link

e.g. myopenid.com/username/patientReport?id=xxxxxxxxxxxxxxxxxxxxx

When clicked, user could be prompted to log in & see the details.

EDIT2: Yes I know, I am moving away from the topic of creating an outlook addin. But why go a difficult way, when a simpler solution can be derived? :)

shahkalpesh
That's the gist of it. But, I want it as automatic and easy-to-use as possible. The message authors needn't worry about the implementation details.
Zack Peterson