Try these in your DotNetNuke Host Settings >>> Advanced Settings >>> SMTP Server Settings:
***The GMail Account you specify will be used when sending email from your DotNetNuke Website.

SMTP Server and port: Enter the SMTP Server Address. You can also specify an alternate port by adding a colon and the port number (e.g. smtp.googlemail.com:587). Enter the SMTP server name only to use default port number (25)	Test
SMTP Authentication: Enter the SMTP Server Authentication method ( default = Anonymous )	Anonymous Basic NTLM
SMTP Enable SSL: Used for SMTP services that require secure connection (usually not needed)
SMTP Username: Enter the Username for the SMTP Server
SMTP Password: Enter the Password for the SMTP Server

Did you figure out how to keep Google Apps from changing the from information?  This is killing me as my ISP stopped allowing local SMTP relay.

You can't.  It's the Google account that is sending the mail.  You need a server you can relay through to do this.

Jeff