Hmm, this has been posted in the FM Server Forum yet appears to have nothing to do with FM Server...
The typical structure to your script would look like this:
Show Custom dialog ["Do you really want to send this email" ]
IF [Get ( Lastmessagechoice ) = 1 //button 1 is "yes" ]
Whoops! Click happy on the wrong forum.
Thank you very much, PhilModJunk, for your helpful advice. Works like a dream.