in 1990 the internet was something talked about on your BBS as a mystical place you couldn't get access to except as a student. Lots of places had email, but it was unusual for anyone to be connected to any other system. Sometimes a BBS was part of fidonet, but that was rare (and there were many many less known networks).

Even if your typical big company did have a gateway for their internal email to other companies, odds are very few people used that gateway for business. If you wanted to communicate with somebody in a different company you dialed the phone or sent a fax.