The settings that are managed through the SMTP Settings feature of the Application tab exist within the section of the Web.config ...

The settings that are managed through the SMTP Settings feature of the Application tab exist within the <mailSettings> section of the Web.config file for the Web site, which is a child of the <system.net> element (not of the <system.web> element). The highlighted lines in the following code are generated after you use the Web Site Administration Tool to specify SMTP settings.