I did this many, many moons ago. It takes a little bit of work, but not too much. If you want to send mass letters that look customized (based on the name of the recipient), here’s how you do it: How to use Microsoft Excel and Word to send multiple emails.
This post was written in 2009 by Walt Feigenson, a friend in the Silicon Valley area. We met when I was in town a few years back, speaking at some job clubs, and the last time I saw him was at his house for dinner (on a different trip). The stories he has of the history of software, which he was involved with, are awesome.
And this merge technique, which might feel a little dated, is really quite powerful. YMMV, based on editions of Word/Excel… if yours doesn’t work the way he describes, figure it out and let me know in the comments what is different (that is a tactful way of saying: I’m not tech support for this tactic – good luck :))