The mailer currently uses message.set_payload method without a charset
(charset is an optional argument). This results in a traceback with a
decoding error:

Error: 'ascii' codec can't encode character u'\xe4' in position 15:
ordinal not in range(128)

The fix is to pass the configured charset.

Note that I'm using mailer.standard_message explicitly in a reactor and
I'm not sure if normal nosy emails use the same code path.

I'll push a patch but wanted to document this.