yum install/update receives error [Errno 14] HTTP Error 500: Internal Server Error

Issue

RHN proxy log shows following backtrace

[error] [client X.X.X.X] PythonHandler proxy.apacheServer::Handler: Traceback (most recent call last):
[error] [client X.X.X.X] PythonHandler proxy.apacheServer::Handler:   File "/usr/lib64/python2.4/site-packages/mod_python/apache.py", line 299, in HandlerDispatch\n    result = object(req)
[error] [client X.X.X.X] PythonHandler proxy.apacheServer::Handler:   File "/usr/share/rhn/proxy/apacheServer.py", line 69, in __call__\n    severity="unhandled")
[error] [client X.X.X.X] PythonHandler proxy.apacheServer::Handler:   File "/usr/lib/python2.4/site-packages/spacewalk/common/rhnTB.py", line 180, in Traceback\n    rhnMail.send(headers, outstring)
[error] [client X.X.X.X] PythonHandler proxy.apacheServer::Handler:   File "/usr/lib/python2.4/site-packages/spacewalk/common/rhnMail.py", line 47, in send\n    (headers, toaddrs) = __check_headers(headers)
[error] [client X.X.X.X] PythonHandler proxy.apacheServer::Handler:   File "/usr/lib/python2.4/site-packages/spacewalk/common/rhnMail.py", line 41, in __check_headers\n    toaddrs = to.split(',')
[error] [client X.X.X.X] PythonHandler proxy.apacheServer::Handler: AttributeError: 'NoneType' object has no attribute 'split'