MSDN seems to indicate that rsreportserver.config is the file that contains this setting.   See:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/rsadmin/htm/arp_configserver_v1_4bzl.asp

It appears that rswebapplication.config just tells Report Services that there is an e-mail extension installed but it does not specify the SMTP server.