Chidi,

I am not sure what you really mean, but there are several steps necessary:

1. Which DNN version are you using? In DNN 9, go to Settings :: Servers, select the tab "Server Settings" and enter the SMTP Server settings. In former versions this is found under Host :: Host settings, tab "Advanced Settings", panel "SMTP Server Settings". When you run different portals in the same installation, you can define different settings for each portal under Admin :: Site Settings, tab "Advanced Settings, panel "SMTP Server Settings".
2. In DNN 9, go to Settings :: Security, tab "Login Settings" and select the Primary Administrator. In former versions this is in Admin :: Site Settings, tab "Advanced Settings", panel "Security Settings" - the setting is called "Administrator" there. The email address of this user will be used for all outgoing emails (registration confirmation, password reset, etc.)
3. When you want to send a newsletter, you can install the module mentioned above. This module was included in DNN up to version 7, and can be found under Admin :: Newsletters there. There are other modules for that case in the market, which are more comfortable and support external services as MailChimp or others. Have a look at the DNN Store.

Happy DNNing!
Michael