Dear Mr F
My apology if I’m infringing on your busy schedule, but
  1. as you know, in our day-to-day business operations Managers are normally only contacted by clients to lodge complaints.
  2. I would, however, like to compliment you and your Company with outstanding service rendered by one of your employees, Mr M, Portfolio Manager.
  3. In the same vain I would thus accordingly applaud Mr M for his professional, proficient and efficient service rendered so far during my dealings with him. Thanks and keep up the good work!!!!!.