In the IDE Sender and SenderFullName are only available in the event Sub and not in Subs called by the event Sub. The reason for this is that they are actually local variables in the event Sub. This is not however true for a compiled application where they are effectively global variables.
Attached is a modified app that does what you want. Rather than try to explain here I've left debugging MsgBoxes in so you can see what is happening. Note that is it is not really a good idea to name controls with the same name as language keywords as this can cause confusion so I have renamed the button.