Here is one possibility:
Send mail only interacts with either the default email client or an SMTP mail server. WHen there is more than one email client installed, this sometimes creates issues in the windows .ini file. At least it does in older versions of FileMaker and I've heard nothing to suggest that this is handled differently in verion 12.
See this thread in a different FileMaker Forum: http://fmforums.com/forum/topic/60795-send-mail-script-step-crashes
yep that sorted it.
win.ini just needs mapi lines adding for the email program
Then works fine.
Thanks for that