I discourage underlinging, and all-cap treatment of words for emphasis. Keep font size to 12 for most business corresondence. Anything less than that is hard to read. Underlining makes reading and scanning of material more difficult. All-cap treatment is discouraged on several grounds.
Research demonstrates that all-cap treatment of letters makes words harder to read.
Both of the practices above are aimed at enhancing emphasis which can be better achieved through changing font, font style, or font size.