The speaker imagines receiving a letter in the evening, written in red ink, and the surprise is who sent it: their own heart. Red is the color of blood and warning marks, so the image suggests something urgent, maybe painful, arriving from inside the body rather than from the outside world.
The second stanza says the letter was written "in illness," and it describes that state as a "sudden back-country," a remote, rough place with "no pleasure" in it. The word "sudden" matters: the illness seems to have dropped the speaker into unfamiliar, lonely territory without warning. We can read the illness as physical, emotional, or both. The poem doesn't say.
The mood is quiet, uneasy, and a little eerie. There is no big outburst. The speaker simply reports the letter, which makes the moment feel stranger and sadder. It's as if the heart has been trying to say something and the speaker can only pass the message along.