Code 500?? pyWM is still beta software. If you should run into bugs, the developer would greatly appreciate if you reported them. There are some things you should know before you report bugs.
Check your error log
Make sure the include the python traceback that you'll find in your Apache error log. This is usually found here:
/usr/local/apache/logs/error_log
but, depending on the OS you're using, it could be almost anywhere.
Your Email
If the email lib choked on a particular email, it may be useful to see the message. Before you send it, make sure to remove any personal information you wouldn't want to divulge--but try to leave formatting intact.