There is a very simple solution (FM ready made solution) you can adapt at start up of my FMP 9 Adv.
It is called Lending Library.fp7 . Have a look in your startup screen (Under Home collection)
It has a simple merge of text and fields.
Another way of doing it is to create a table with messages templates and another table with distribution list and link your distribution list to your clients table.
This is a more sophisticated approach (I'm still trying this since I'm a beginner with FMP)
Sorry If I confuse you more than answer your question.