Yep, as I said, it was pretty standard for timesharing... and certainly IBM, DEC and most of the BUNCH companies had something like it in their 1960s developed systems.
If communication within one system is not
recognized as email, then the exchange that
opened in Boston in 1877 was not a
telephone system.
Amen...
It's all about MetCalfe's Law and the value of the messaging system with the # of users. The ARPANet, like the connected exchanges, increase the number of #s.