This is odd, I wonder are there 2 email clients on this PC?
Maybe express, Mozilla or something else in the way?
Also look in the mail profiles to see if anything is amiss.
control panel > mail.
Also, make sure the user is logged in with the appropriate security, I would just make them a full administrator. It is possible they don't have access to the profile.
I would be certain though its nothing to do with Filemaker.
Mail Profile seems good.
I can't make the user full administrator or not--but this is where I think we're on to something. It could be a result of "Programatic Access" at a group level. I'm going to try my FileMaker test file with another user that, previously, always had admin permissions. I'm also looking for another application that would open up outlook too. Ultimately, I have confidence that it was their IT department's fault, but I have to go down every alley first.
I have had this happen when one of the address fields was empty. I had to use a conditional statement to set the CC: or BCC: field to "" instead of the contents of an empty field.
There are some other posts out there about this same issue. Look in the "More Like this" section on the right side of the screen. Someone else may have found a solution.