It can't be done with a SINGLE click, but the instructions for getting messages by email are here: http://vps.xlhnetwork.org/~xlhforum/forum/index.php?topic=149.0
The instructions look complicated, but mostly it's just because there are so many different options. It only takes a minute or two to set it and then you can forget it. First, you choose one thread to follow, set your defaults for how to get the messages, and then you can click as many (or as few) other threads as you wish.
Another option, rather than getting messages by digest, is just to make a habit of checking in once a week or once a month or whatever frequency works for you. (Some people find it overwhelming to get messages every day or even every week, so this option is good for those who want to control when they want to be involved.) From the main page, scroll down to the bottom, and you'll see a link for "recent messages," which you can then scroll through.