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?