From the web.config? Like this
On your web.config:
<appsettings>
<add key="smtpServer" value="mail.domain.net" />
<add key="smtpPort" value="25" />
<add key="from" value="noreply@domain.net" />
<add key="mailUser" value="user@domain.net" />
<add key="mailPassword" value="password" />
</appsettings>
On your code C#
var mailUser = ConfigurationManager.AppSettings["mailUser"].ToString();
var mailPassword = ConfigurationManager.AppSettings["mailPassword"].ToString();
var from = ConfigurationManager.AppSettings["from"].ToString();
var smtpServer = ConfigurationManager.AppSettings["smtpServer"].ToString();
var smtpPort = ConfigurationManager.AppSettings["smtpPort"].ToString();