I remember back when I was at school, we were on RMNet - CC3 possibly. It was Windows 98 frontends and NT4 backend anyway.
Lets see.
All the shutdown screens on all the machines were changed to advertise our Unreal Tournament clan
On occasion various people would change put c:\con\con in the startup folder
Email attachments came out of your 5MB or whatever we had of file space. But when your space was full, it wouldn't let you log on until you'd freed up space. Which you couldn't do till you logged on.
Hence the deluge of emails with massive attachments called things like "Say goodbye to your login"
There were plenty of other shenanigans too, but I only have so many hours in the day...
TL;DR - I'm not surprised that the mail system was insecure.