I've heard that interpretation. The US post office has a government-granted monopoly on delivering mail. There was an exception in the original law for "extremely urgent material", which allowed Fedex to come into existence [0]. If Fedex hadn't been tough (and beloved by law firms who needed a lot of overnight delivery) they might well have been sued out of existence by the USPS.

So early users of email tried to be careful to not too obviously use it a replacement for postal mail.

[0] https://www.quora.com/Why-is-the-US-postal-service-considere...