+----------+    +----------------------+     +-----------+
| Sylpheed |----| libsylpheed-plugin-0 |--+--| Plug-in A |
+----------+    +----------------------+  |  +-----------+
  Sylpheed         Plug-in interface      |   Plug-in DLL
                   library             +--+
       |        +------------+         |  |  +-----------+
       +--------| libsylph-0 |---------+  +--| Plug-in B |
                +------------+               +-----------+
                LibSylph mail library