For some of my naval officer contemporaries. I don't know why you didn't learn this at the USNA, in your NROTC program or at officer candidate school.
As a rule, EVERY letter, unless insulting in its character, requires an answer. To neglect to answer a letter, when written to, is as uncivil as to neglect to reply when spoken to. In the reply, acknowledge first the receipt of the letter, mentioning its date, and afterwards consider all the points requiring attention. From @artofmanliness