I read an email. Then I started asking myself "What's the difference between so and really?" I couldn't answer my own question with my background knowledge.
The intensifier so relates to a gradable use of an adjective whereas really goes with an absolute reference.
He drives so fast.
This refers to "how fast" (gradable).
He drives really fast (absolute).
This refers to the fact that he drives fast.
However, in contemporary usage, both writers and readers are accustomed to the words being interchangeably used very often, especially by the young.
I might add that the adverb 'really' in 'He drives really fast' is similar to 'very'.
And with 'so', you can also add a result, as in '....so adverb/adjective that...."
He drives so fast that I'm afraid to get in his car. I'm so hungry, I could eat a horse. It's so cold that I can see my breath when I exhale.