Formal* not forum, apologies.

Well in a forum setting like a business letter, we usually start greetings using the last name of the parties involved, if we don't know this who is involved we usually say "Good Morning, Afternoon, or Night" (if you work at night). We can also use "To whom it may concern:" and then continue with the body of the letter. I guess he was upset at how informal the letter was. We don't usually say "hi" even if you know that person or your friends, your not suppose to correspond in an informal tone for offical letters. Next time try : "Good (insert time of day) and continue with the body of the letter or Good (insert time of day) Mr. (Last name) and Mr. (Last name). Usually if it's two unaffiliated persons (like people who are not married or share the same last name) we just say give a regular greeting (good afternoon etc.) and continue with the body of the letter. If it's a married couple you would say "(Greeting) Mr. and Mrs (last name). Do you understand? I know that was a a lot...and much later than you asked but I hope it helps.

What did he say that made him seem angry? I doubt he would've tried to be funny if he really was angry.

I started the email with" Hi Both...", then our boss sent back and said "who is both, I don't know that person..." seems we can't use "both" like that, I thought it just mean "两位" in Chinese before, but now it seems not.