Thanks for the link! It looks like this affair over webmail sparked a lot of investigation and resulted in a firing, two years probation, and a $100,000 fine. That sounds like a great start for this kind of thing and they were just talking dirty, not actively sharing mission details!

Overall, yes let's please investigate and appropriately punish wrongdoing at all levels.

Right. They also use cell phones and send emails. The difference is, from the Snopes article:

"... it explicitly did not allow use of Signal to communicate "non-public" Department of Defense information, which would have included the conversations Trump administration officials had in their group chat."