[Sparkle]

Well my reply is a slight digression, but I think perhaps it within the context of the OPs question...

90% of the time I refer to my colleagues by their first name, even supervisors, however, there are exceptions.

When I am meeting with donors and discussing the university to which they are an alma mater of, I will always address the head of school as "Chancellor soandso" or "President soandso" or "Dean soandso".

They may have been John and Bill and Lee to me, but in a professional capacity when discussing the upper echelon of university administration in the formal context of philanthropy, I use their titles. It legitimizes the leadership with the donor and it creates a sense of cache between the donor (alum) and the leadership.

But on a one-on-one working basis, I almost always address someone by their first name.

When I worked abroad, however, I was given a crash course in titles, particularly the importance of ceremonial titles...

I learned to address people as:

The Right Worshipful, The Lord Mayor of Lambeth
The Very Reverend, The Dean of Christ Church

as examples. It usually made me laugh to address a letter or email such, but it was expected in formal correspondence.

Once, by accident, while formatting a letter to be sent to all of my donors (alumni of Christ Church College, Oxford), I slipped up and never caught an important edit between the mail merge autofill salutation and my drafted salutation -- I ended up sending over 500 letters to the upper echelon of alumni addressed with a double salutation of "Dear Dear":

"Dear Dear Sir John"
"Dear Dear the Worshipful the Mayor of xxx"
"Dear Dear Mr Cromwell"
"Dear Dear Sir William" etc...........

Ironically, I raised more money from that appeal than any other letter campaign in my time at that company, and I had copious notes from the gentlemen saying how gracious and lovely I was. (because they perceived my double salutation "dear dear" as the height of obsequiousness).

Titles are a funny thing.