You are sending to the same address when testing preview?
When you send it it shows the progress bar and shows that it completed?
You are not seeing any clues in the mojoPortal log?
What version of mojoPortal? What version of .NET framework? What database platform?
The only other thing I can think of is if you disabled the newsletter, for example if I'm working on my local machine with a copy of the database from this site I have a setting to make sure I don't accidentally send a news letter from my development machine by adding this in my user.config to disable it:
<add key="NewsletterTestMode" value="true" />
With that setting the newsletter task just goes through the motions but doesn't actually send the email.
If the taskqueue is disabled that would also prevent the newsletter (since it depends on the task processing) but then you would not see the progress bar showing completion.
Hope that helps,