Though the use of a “Dear” in business letters seems informal, it is a widely accepted way to address managers or directors. Of course, Dear dad implies a level of familiarity and conveys a more personal approach.
The following extract from The Grammarphobiaexplains how the usage of “Dear” in business and personal letter evolved:
The tradition of using “dear” in letters dates from the mid-15th century.
It was first recorded, according to OED citations, in a letter beginning “Right dere and welbeloved,” written in 1450 by Queen Margaret of Anjou, wife of King Henry VI.
As the OED says, uses of “dear” in letters—as in “Dear Father,” “Dear John,” and so on—“are still affectionate and intimate, and made more so by prefixing My.”
But, Oxford continues, “Dear Sir (or Dear Mr. A.) has become since the 17th c. the ordinary polite form of addressing an equal.”
And not just equals.
Our handy copy of Miss Manners’ Guide to Excruciatingly Correct Behavior, by Judith Martin, recommends that letters to dignitaries include greetings like “Dear Governor Stately,” “Dear Mr. President,” “Dear Mayor Tuff,” and (to a corporate bigwig) “Dear Mr. Pious.”