When a letter appears within a collection which consists entirely of letters, or in a collection consisting of letters, essays, and other short works each of which can be encoded as a separate <div>, the WWP will use <div type="letter">. Within <div>, most letters will contain an <opener>, body paragraphs, and a <closer>.
Note that <opener> and <closer> have the same content model, to accommodate the fact that some letters have the dateline and similar information at the end rather than at the beginning. Note as well that <signed> is not only for actual signatures, but for printed names as well.
Note also that the TEI currently does not provide in any satisfactory way for postscripts; the WWP uses the WWP element <ps>.