We had a similar issue with our first email campaign. We had 1833 subscribers, of which it shows 0% failure rate, but yet only 181 delivered for a 10% success rate. We have no idea at this time what happened to the other 90%.
We are using an exchange server as a mail relay, and it looks like sitefinity may have not sent all 1183 subscribers an email. Of the 181 that are delivered, they are mostly at the top of the alphabetized email list. We determined that the exchange server has a limit of 1000, for the particular user.
If there are any users of Sitefinity that have been using the email campaigns successfully, it would be helpful to understand if they have run into similar issues and have resolved them.