must have been lost in the mail 
(ROOT (VP (MD must) (VP (VB have) (VP (VBN been) (VP (VBN lost) (PP (IN in) (NP (DT the) (NN mail))))))))
(ROOT (VP (MD ) (VP (VB ) (VP (VBN ) (VP (VBN ) (PP (IN ) (NP (DT ) (NN ))))))))