Hi Steve,
The problem is that doing this will not reduce maintenance of the Web.config during upgrades. If we specify an external file, then we must include that file and it would overwrite a file of the same name during upgrades so to use a custom file you would still have to edit the Web.config to point to the custom file and this is the same amount of effort as just maintaining the smtp settings directly in Web.config. In fact having more files to worry about during upgrades I think would cause more confusion and more support issues.
It would be nice if it worked like the <appSettings where you could point to a different file that will override if the file exists but keep default settings right in Web.config, but it doesn't work that way.
Best,
Joe