I use SerialMailer:
which seems to do the job I need. I want to typically send 50 to 100 emails at a time, personalised. I think it can do html mail too, but I haven't tried that.
I looked at the three found at the Mac App Store. The first and last ones did not really seem to be mail merge. The middle one is a bit pricey, so I would want to test it first. I found the developer's site and they seemed to have a download trial, but I could not get it to download.
But SerialMailer seemed exactly perfect for what I wanted to do. I could preview, use formatted emails, and even track the progress as the mails were sent out. And they all arrived just fine. And it's only $29.95. I believe I will go with that.
Thanks for your help, people.
For anyone who did end up using SerialMailer, I have a question: does it let you change the reply-to?
My office is trying to send out a mass email to alumni, and we would like to send it from my boss's address but have any replies or bounce-backs come to my address. Is there a way to do that with this program?