This happens if the mime_encoding in trac.ini is set to none
2009-07-21 16:19:39,663 Trac[api] ERROR: AnnouncementSystem failed.
Traceback (most recent call last):
File "build\bdist.win32\egg\announcerplugin\api.py", line 369, in _real_send
distributor.distribute(transport, packages[transport], evt)
File "build\bdist.win32\egg\announcerplugin\distributors\email_distributor.py", line 204, in distribute
formats[format], None, to, public_cc)
File "build\bdist.win32\egg\announcerplugin\distributors\email_distributor.py", line 287, in _do_send
dummy = body.encode('ascii')