When is Mr/Mrs appropriate?

I often receive business emails starting with "Dear Daniel..." or "Hello Daniel..." although I haven't been into contact with the sender before. As an Austrian citizen (thus german speaking) this is quite unusual to me, I always write "Dear Mr. Smith..." or "Hello Mr. Smith...".

When is it appropriate to address someone with Mr/Mrs instead of his given name?

Very good question! Since the English-speaking world has become much more informal due to the influence of pop culture, most people use first names almost exclusively. My mother insisted on being called Mrs. by everyone younger than she was, even though many of my friends would have called her by her first name. She made it clear she did not want that.

I think Brett meant by honorific Dr. or other such title, not Mr/Mrs/Ms. But I may be wrong.

What would I, who am a person of formal upbringing, do? First, I check how the person has signed themselves in any prior correspondence (Best regards, Sally). If they use only first name, so do I. In cases where I am writing someone for the first time, if I know they are older I use Mr/Mrs/Ms. If they are not older, I will use Mr/Mrs/Ms and their family name. In cases where family name is not obvious – in cultures where the family name may be written ahead of the given name – I use the whole name. (In your personal case, I would have used Daniel! This is a very interesting and thorny question.)

The bottom line is, it would never be badly viewed in any situation if you use Mr/Mrs/Ms - you might be thought stuffy or too formal, but that's preferable in my opinion to being seen as lacking in respect. Are you more thoroughly confused now?

This is really an etiquette question, not English. The etiquette for letters is fairly well known (which is not to say you cannot ignore it if you choose), but email is still not old enough for it to be clear whether you should start with'Dear Bill', 'Bill' or no salutation at all, even when you are addressing a friend: much less how to address a business contact or potential contact.

In addition, email makes it easy to discover first names, and some people regard it as impolite not to make the effort to use them, even if you have never met the addressee. Others still prefer the old convention of calling somebody 'Mr Smith' (or just 'Smith', though that too may be perceived as insulting) until invited to use the first name. Add in the fact that many contacts want to be perceived as a friend of yours because they want a favour or just hope you will become a customer. Add in also the worldwide nature of email, which is bound to cross lines of national or cultural politeness, and the whole area is rife with confusion; I myself, if I feel slighted or confused, try to make allowances for the sender's lack of perfect English (because, obviously, I am the only writer who is perfect).