If you need to authenticate, install acs-mail-lite from HEAD. This has in the complex_send function a parameter for Authentication (both to be passed when calling as well as a parameter in acs-mail-lite parameters). NOTE: You have to install TCLLIB for this to work.
Also you may want to check on the configuration of your local mail server. OpenACS will use your mail server to send emails out so if you can't reach any domain but yours, it's probably related to the configuration of your local mail server.