You have to select the client in fmp.
Your developer should know this, they had to set it up to begin with.
It crashes because it setup for outlook and outlook is not installed. You will have to locate the script that sends the email and changes the settings to the new client.
I believe that you will have problems using Outlook (the security check, for example), and a few more when you don't. I got so tired of it that I created a gmail account and send all mail through it. It worked first time, and every time, and I've found no downside, so I just do that every time now.