Why are some words replaced by symbols?

For some unknown (to me) reason some words are replaced by symbols. I sent a message and one word (unfortunately I've forgotten what) was replaced but what looks like a coffee cup. I know that Pandora will change emoticons [I just received an email and Pandora replaced a smiley face with what looks like a radio telescope or maybe a TV dish.] but I don't use emoticons and I'm sure the cup replaced a regular word. Another message I received showed 3 books but I don't know what that was for either. There must be a setting for this that I haven't found? Thanks.

Those are emoji-characters. They are tied to particular font i.e. not all fonts support emoji.
Try change font to Segoe UI, Segoe UI Symbol or Segoe UI Emoji.