The Collaborative International Dictionary of English v.0.48:

Posthouse \Post"house`\, n.
   1. A house established for the convenience of the post, where
      relays of horses can be obtained.
      [1913 Webster]

   2. A house for distributing the malls; a post office.
      [1913 Webster] Posthume

WordNet (r) 3.0 (2006):

posthouse
    n 1: an inn for exchanging post horses and accommodating riders
         [syn: posthouse, post house]