Email at that time could be of several different kinds, not all of which utilized the now infamous letter '@'. For instance, some people were on BITNET, and in order to reach them from the ARPA-net (that is, the seed of today's Internet) one had to type in addresses like john%node1@bitnet.net.
The UUCP mail transfer protocol was also quite common, meaning mail delivery could take several days because systems only exchanged information (through a dial-up connection) once a day (or night).
There were other nets as well. FIDOnet was the predominant means of communication between users of BBS:es.