In the letter, her longing for her old friend leaks out of every line.
Tips
usage
Subject is always an emotion (喜悦, 悲伤, 思念, 感激). Literal: '(the feeling) overflows beyond the surface of the words' - the speaker tried to be measured but the emotion still leaks through.
register
Written/literary flavor. Essays, letters, formal reports. In spoken Chinese prefer 看得出来 or 一眼就能看出.
