They'll always be able to give you new insight. If you're a male writing to a majority of women, have a couple of women in the office read the letter to see if it fits the way they think and feel. The reverse is true as well. If you already have a client in the target market to which you are writing, as them to review it for content, clarity and applicability to them. Quiz them to see if they would respond favorably to your points.
Research shows that block style is difficult to read.
By indenting you make it easier to read, it sets off the points, and calls attention to changes in direction. It is far better than simply underlining or using bold face type. Keep your sentences short. Kenneth W. Houp and Thomas E. Pearsall conducted research on sentence length. Keeping your sentences short promotes understanding and response.