Mail merges are now possible using what are called markmeld "multi-output" targets. Basically, you need to specify 2 things: 1) a variable with an array to loop over, and 2) a variable name that will assume the value of each item in the array. For a mail merge, your array would be a list of addressees, and you'd assign each one to some variable, say,
recipient. Then, you just have to ensure that your jinja template uses the
recipient variable, and specify an output filepath that uses something that varies in the
recipient variable, and that's it.
This process is documented here:
See this issue for more information: https://github.com/databio/markmeld/issues/1.