I think it would be great to define a smiley per message, thus when you receive a message that has a smiley set in the X-tag, it would appear in the message list: there is already a "Subject", a "From" and a "Date" column, we could add a column for the smiley associated to the message if the sender chose one.
Also, enabling smileys inside messages would be great, like Opera does in its mail client.