Should I address an EiC by his given name after he undersigned an email with it? An EiC very politely answered an enquire of mine via e-mail. He signed with his given name. In case I write to him again, should I keep greeting him as "Prof. [His surname]"? That strikes me as perhaps a little too stiff. On the other hand, calling him by his given name sounds way too familiar to me. What is the etiquette in this type of cases?
Dealing with a superior is always tricky. And I would say, replying with his first name does not give everyone a pass to just address him like this. 

A few things to consider are:


How well do you know him
Have you done work with him
How does he addresses you in the reply
How do other people of your social status address him


My rule in a general business is to follow how the person is calling me. If they use my family name or first name, or even use my title or not. Copying this form of address is usually safe. 

With a superior, my default would be to use at the very least his family name, and even throw the title in it if I want to show deference (especially if he's doing me a favour). Now, if you know that person and have a good working relationship with him, it might come a point where you want to just skip the formalities and be on first name basis. That's a judgement call and I don't think we can help you with this, as it comes with your personal experience with that particular person. Furthermore, some people are really tight on with being called by their full titles.

In the end, if you are unsure and it is expected that you will have an extended correspondance/work relationship with him in the future, I would just advise you to just ask him if he would be OK with it, and keep using the formal titles until then. The worst that could happen is you're gonna look stiff/strict/pedantic about etiquette maybe... it's not as bad as being painted disrespectful.