There wasn't a use for @ in addresses at the time.? IIRC the @ sign was even defined as chardel, in linemode that is.? Dates from typewrite times.? When you'd type an @, the preceding character was ignored.? In some release the default SYSTEM CONFIG turned LINEDEL and CHARDEL OFF
One could send a message to another VM system using the TELL EXEC? ? ? ?TELL myfriend AT otherVM hello Or, use the native RSCS commands ? ?CP SMSG RSCS MSG otherVM myfriend hello The TELL EXEC consulted also "userid NAMES" so your addressee could even be a list of people or a nickname; transparently on the same VM system or via RSCS to a remote VM system