failobj has to be a list or the `+' can fail.
\begin{verbatim}
from email.Utils import getaddresses
-tos = msg.get_all('to')
-ccs = msg.get_all('cc')
-resent_tos = msg.get_all('resent-to')
-resent_ccs = msg.get_all('resent-cc')
+tos = msg.get_all('to', [])
+ccs = msg.get_all('cc', [])
+resent_tos = msg.get_all('resent-to', [])
+resent_ccs = msg.get_all('resent-cc', [])
all_recipients = getaddresses(tos + ccs + resent_tos + resent_ccs)
\end{verbatim}
\end{funcdesc}