This Action can’t do that. You’ll need something that has additional configuration options. I don’t know of anything in an Action that can do this. Either use a different SMTP host (one that doesn’t care, like the free domain e-mail that comes with most hosting packages) or you’ll need to hire a programmer to configure one of the many PHP mail packages* that can do this, and have them write a handler script similar to the one that Send Form writes that will hook into that package.