No. "Dear sir" sounds like something he picked up in high school English class, taught for use when addressing an unknown recipient.

If writing this in Finnish you'd normally use something very close to "Dear Mr Reynolds".

Most of us rarely write such formal letters. I don't remember when is the last time I've written something as formal for a Finnish recipient.

At school I was taught to use "Dear Sir or Madam" or "To whom it may concern" but neither of those sound natural, they're just learned. If writing for a Finn, I just start with "Hei" ("Hey") (in fact that has lead me to start emails in English with just "Hey" as well). I may have used some other formality once or twice in my life when it was required for school or such matters but not as an adult.

So it may be that they were out of their element when writing that email and just put the first thing that came to their mind.

"Hei" means "hi". You can use that in your email.

English "hey" is said when someone is annoyed. "Hey you! Stop that!"

Hey is not exclusively used when annoyed. It's very commonly used as an informal greeting.

Hmm, tone is what differentiates the greeting and the admonishment. Your 2nd sentence is correct, but not exclusively.

TL;DR I use "hey" as a greeting.

[en-gb native]

But it's very informal/slang, used among peers. e.g. If I answered the phone to an unknown caller I'd say 'Hello?', but a friend might get a 'Hey.'

I certainly wouldn't say 'Hey IANA, please reserve me port 22' any more than I would sign off 'kthxbye'.

[also en-gb native]

> "hey" as a greeting

I blame the US TV series "Friends" for that.

Curiously the OED doesn't (yet) have this greeting sense.

> Curiously the OED doesn't (yet) have this greeting sense.

Interesting, neither does my (more liberally accepting than Oxford) 2016 Collins. I've certainly been using it as an informal greeting since '00s.

I don't know if I got it from Friends, it wouldn't have occurred to me, but I did start watching it around that time so it's certainly plausible.