It's quite common in US to refer to people by their first name perhaps an attribute of low context culture. I particularly like it having come from a high context culture as it reduces friction and gives people the confidence to voice their opinions irrespective of their social strata.
I'm guessing Chris knows Satya on some personal level.
Maybe. And I get that as CEO's they can be regarded as peers, but if you're going to call this a "letter", I think it would have been better to use either full name (which is how it's signed BTW), or Mr.
This is not some random guy but another CEO.
Who used HIS full name in the signature. At the very least he should have use MS CEO's full name.
I confess that I use this same pattern frequently for work emails outside the company. Inside the company, I only sign with my given name.
[GivenName],
[Body]
[Closing],
[FullName]
Usually people use their full name in signatures. Still don't see what the issue is.