Build a GEFX graph file of who has sent mail to whom.
First, get MBOX-formatted files of the mailbox(es) you want to examine. This can usually be accomplished by googling with the term "export mbox" and whatever mail client you use. Then, install the dependencies for the script and pass it the files you want to examine:
virtualenv --python=python3 venv
source venv/bin/activate
pip install -r requirements.txt
python mailgraph.py -o outputfile.gexf file-to-parse.mbox another-file.mbox
The script will then churn through the files you provided, and output outputfile.gexf. This can be loaded using Gephi and explored. Gephi is awesome, and makes it incredibly easy to poke through this data. Poke through the list of features if you're curious about what you can do with Gephi. (You could use other software too - GEXF is a standard format.)