Good news, I got it to work! It was my own fault. Thanks to your remark about the native PHP mail function being used.
I remembered yesterday during another WAMP setup that I didn't input my mailserver details in this setup.
I looked in C:\xampp\php\php.ini, found the following and changed it to my own mailserver address:
; For Win32 only.
SMTP = localhost
smtp_port = 25
So maybe this helps other users with email problems. But thanks for your effort to help solve this Thorsten.