I really appreciate this module, and I use it intensively for mailing to students and colleagues. My typical mailing size is 20, driven by a rather complex CSV configuration file with a dozen of columns. It works perfectly.
I have a suggestion: adding a mean to exclude certain lines of the CSV file to be included in the processing. It could be for instance is some variable is defined. This would enable partial mailing much more easily than today.
This review is for a previous version of the add-on (4.6.0).