I would like to check on this usage when addressing an unknown person who might be male or female.
For some time I was in the habit of using "Dear Sir or Madam", but was told (by a woman recipient, who was a lecturer in English) that "Dear Sir" was correct. Since then I have used that, though uneasily at times.
Any informed comments would be appreciated.
Use what you find comfortable. There is no problem with Dear Sir or Madam - save in the perceptions of individuals.