Google apps email is free, fast and offers a lot of cool features. It also allows more smtp relays. Here are the steps of setting up a google apps email account and using google's smtp server for your dnn site. The standard account is free with up to 50 email accounts.
1. Go to: http://www.google.com/a/cpanel/domain/new
2. Enter your domain and follow the steps
3. Verify your account
4. Under Dashboard page, go to the Email -> Click "Activate email"
5. Change MX records
6. Click "I have completed these steps". You will be redirected to the Dashboard page. You will see "Updating... We are checking MX records for your domain. This may take 48 hours to complete." In fact it often only takes a few minutes.
7. Once the email is active, click the email account you have, under settings, go to "Forwarding and POP/IMAP, by default, pop is enabled, you need to enable "IMAP".
8. To set up the email client, use the following info ( or refer to google's page to use IMAP instead of POP3):
SMTP PORT: 465
POP3 PORT: 995
Check SSL for both
To configure dotnetnuke smtp server